Heat exchanger for a rankine cycle in a vehicle

A vehicle includes a Rankine cycle containing a working fluid for waste heat recovery and has an evaporator. The evaporator has a heat exchanger tube positioned for generally horizontal flow of the working fluid therethrough. An inlet header is connected to a lower surface of an end region of the tube. An outlet header with a plurality of risers is positioned for generally vertical flow of the working fluid. The riser headers are connected to and spaced apart along an upper surface of the tube.

TECHNICAL FIELD

Various embodiments relate to a heat exchanger for transferring heat to a working fluid in a Rankine cycle in a vehicle.

BACKGROUND

Vehicles, including hybrid vehicles, have internal combustion engines that produce exhaust gases at a high temperature. The vehicle also may have various systems with waste heat that require cooling, e.g. the engine coolant system with coolant fluid. A thermodynamic cycle such as a Rankine cycle may be used to recover waste heat within the vehicle during operation and provide power to the vehicle using a heat exchanger. Due to the mixed phase of the working fluid within the heat exchanger, prior, conventional heat exchangers may result in uneven or non-uniform heating, thermal fatigue and wear, and vapor lock in the cycle.

SUMMARY

In an embodiment, a heat exchanger is provided for a vehicle Rankine cycle having a working fluid. A heat exchanger tube is positioned for generally horizontal flow of a working fluid therethrough. An inlet header is connected to a lower surface of an end region of the tube. An outlet header with a plurality of risers is positioned for generally vertical flow of the working fluid. The riser headers are connected to and spaced apart along an upper surface of the tube.

In another embodiment, a vehicle is provided with an engine. The vehicle also has an expander, a condenser, and a pump in sequential fluid communication in a closed loop containing a evaporator configured to transfer heat between exhaust gas from the engine and a working fluid. The evaporator comprises a generally horizontal heat exchanger tube with a lower surface connected to an inlet header and a plurality of risers connected to and spaced apart along an upper surface of the tube.

In yet another embodiment, a once-through evaporator for a thermodynamic cycle is provided. The evaporator contains a working fluid in a vehicle for energy recovery from waste heat. The evaporator has an inlet manifold with at least one inlet header having an inlet riser section providing a vertical flow component for the working fluid containing a liquid phase. The evaporator has at least one heat exchanger tube having a first end region and a second end region. The heat exchanger tube has an outer surface defining a flow channel for the working fluid. The outer surface is adapted to contact a waste heat fluid. The inlet riser of an associated inlet header is connected to a bottom side of the tube adjacent to the first end region and is in fluid communication with the flow channel to provide the working fluid containing the liquid phase to the tube. The evaporator also has an outlet manifold with at least one outlet header having a collection tube and a series of outlet risers each providing a vertical flow component for a vapor phase of the working fluid. The outlet risers of the outlet header are connected to a top side of an associated heat exchanger tube and spaced apart along a length of the heat exchanger tube between the first and second end regions. One outlet riser is adjacent to the first end region and another outlet riser is adjacent to the second end region. The outlet risers are in fluid communication with the flow channel to provide the vapor phase of the working fluid to the collection tube and the outlet manifold.

Various examples of the present disclosure have associated, non-limiting advantages. For example, a heat exchanger for a Rankine or other thermodynamic cycle in a vehicle is provided. The heat exchanger has heat exchanger tubes or chambers for evaporation of a working fluid in the cycle using a waste heat fluid, such as an exhaust gas flowing around the heat exchanger tubes. As the working fluid is evaporated in the heat exchanger tubes, the vapor phase of the working fluid separates from the liquid phase and rises in vertical outlet risers of the outlet header. The liquid phase of the working fluid remains in the heat exchanger tubes and continues to be heated by the waste heat fluid. The remaining liquid in the heat exchanger tubes has a high thermal conductivity and high thermal transfer efficiency compared to the vapor phase. The design of the heat exchanger results in the liquid chambers and gas pipes having a generally even temperature distribution since phases of the working fluid separate as they evaporate.

DETAILED DESCRIPTION

As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. Description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description, and does not necessarily preclude chemical interactions among constituents of the mixture once mixed. A fluid as described in the present disclosure may refer a substance in various states or phases including to vapor phase, liquid phase, mixed vapor/liquid phase, superheated gases, sub-cooled liquids, and the like.

A Rankine cycle may be used to convert thermal energy into mechanical or electrical power. Efforts have been made to collect thermal energy more effectively or from more than one system that rejects waste heat in the vehicle such as engine coolant, engine or transmission oil, exhaust gas recirculation (EGR) gases, exhaust gases, etc. The present disclosure provides for a Rankine cycle with a heat exchanger or an evaporator that provides for phase separation as the working fluid evaporates, thereby increasing the cycle efficiency and maintaining a generally even temperature distribution of the liquid and vapor phases of the working fluid within the evaporator.

FIG. 1illustrates a simplified schematic of various systems within a vehicle10according to an example. Fluids in various vehicle systems may be cooled via heat transfer to a working fluid within heat exchangers of a Rankine cycle, and the working fluid is in turn cooled in a condenser of the Rankine cycle using ambient air. The Rankine cycle allows for energy recovery by converting waste heat in the vehicle10to electrical power or mechanical power that would otherwise be transferred to ambient air as waste heat.

The vehicle may be a hybrid vehicle with multiple sources of torque available to the vehicle wheels. In other examples, the vehicle is a conventional vehicle with only an engine, or is an electric vehicle with only electric machine(s). In the example shown, the vehicle has an internal combustion engine50and an electric machine52. The electric machine52may be a motor or a motor/generator. The engine50and the electric machine52are connected via a transmission54to one or more vehicle wheels55. The transmission54may be a gearbox, a planetary gear system, or other transmission. Clutches56may be provided between the engine50, the electric machine52, and the transmission54. The powertrain may be configured in various manners including as a parallel, a series, or a series-parallel hybrid vehicle.

The electric machine52receives electrical power to provide torque to the wheels55from a traction battery58. The electric machine52may also be operated as a generator to provide electrical power to charge the battery58, for example, during a braking operation.

The engine50may be an internal combustion engine such as a compression ignition engine or spark ignition engine. The engine50has an exhaust system60through which exhaust gases are vented from cylinders in the engine50to atmosphere. The exhaust system60may include a muffler for noise control. The emissions system60may also include an emissions system, such as a catalytic converter, particulate filter, and the like.

The engine50also has a coolant system62. The coolant system contains an engine coolant fluid, which may include water, glycol, and/or another fluid, to remove heat from the engine50during operation. The engine50may be provided with an internal or external cooling jacket with passages to remove heat from various regions of the engine50using the recirculating engine coolant fluid. The coolant system62may include a pump and a reservoir (not shown).

The vehicle has a thermodynamic cycle70. In one example, the cycle70is a Rankine cycle. In another example, the cycle70is a modified Rankine cycle, or another thermodynamic cycle that includes a working fluid transitioning through more than one phase during cycle operation. The Rankine cycle70contains a working fluid. In one example, the working fluid undergoes phase change and is a mixed phase fluid within the system that it exists as both a vapor phase and a liquid phase. The working fluid may be R-134a, R-245, or another organic or inorganic chemical refrigerant based on the desired operating parameters of the cycle.

The cycle70has a pump72, compressor, or other device configured to increase the pressure of the working fluid. The pump72may be a centrifugal pump, a positive displacement pump, etc. The working fluid flows from the pump72to one or more heat exchangers. The heat exchangers may be preheaters, evaporators, superheaters, and the like configured to transfer heat to the working fluid.

The example shown has a first heat exchanger74, which is configured as a preheater. A second heat exchanger76is provided, and may be configured as an evaporator. In other examples, greater or fewer heat exchangers may be provided downstream of the pump72. For example, the cycle70may be provided only with heat exchanger76, or may be provided with three or more heat exchangers to heat the working fluid. Additionally, the heat exchangers downstream of the pump72may be arranged or positioned in various manners relative to one another, for example, in parallel, in series as shown, or in a combination of series and parallel flows.

The heat exchangers74,76are configured to transfer heat from an outside heat source to heat the working fluid within the cycle70and cause a phase change from liquid to vapor phase. In the example shown, the heat exchanger74is configured to transfer heat from the engine coolant fluid in coolant loop62to the working fluid in the cycle70. The temperature of the engine coolant is therefore reduced before returning to the engine50to remove heat therefrom and heat exchanger74acts as a heat sink in the coolant system62. The temperature of the working fluid of the cycle70is likewise increased within the heat exchanger74.

In other examples, as discussed in greater detail below, the heat exchanger74is configured to transfer heat to the working fluid of the cycle70from another fluid in a vehicle system, including, but not limited to, an engine lubrication fluid, a transmission lubrication fluid, and a battery cooling fluid. In a further example, multiple preheating heat exchangers74are provided and are each in fluid communication with a separate vehicle system to receive heat therefrom. Valving, or another flow control mechanism may be provided to selectively direct and control flow to the multiple heat exchangers.

In another example, the heat exchanger74is positioned downstream of the heat exchanger76such that it is configured as a superheater, and transfers heat from a fluid from various vehicle systems, including, but not limited to, exhaust gas recirculation (EGR) flow. The heat exchanger74provides a heat sink for the EGR flow, and thereby provides waste heat to the working fluid in the cycle70. The positioning of the heat exchanger74relative to heat exchanger76may be based on an average temperature or available heat in the waste heat fluids of the vehicle systems.

A second heat exchanger76is also provided in the cycle70. The heat exchanger76is configured to transfer heat to the working fluid of the cycle from exhaust gases in the engine exhaust system60in one example. The engine exhaust system60may have a first flow path78through or in contact with the heat exchanger76. The engine exhaust system60may also have a second, or bypass, flow path80to divert exhaust gas flow around the heat exchanger76. A valve82may be provided to control the amount of exhaust gas flowing through the heat exchanger76, which in turn provides a control over the amount of heat transferred to the working fluid, and the temperature and state of the working fluid at the exit of the heat exchanger76or upstream of the expander84.

At least one of the heat exchangers74,76is configured to transfer sufficient heat to the working fluid in the cycle70to evaporate the working fluid, as discussed further below. The evaporator receives the working fluid in a liquid phase or liquid-vapor mixed phase solution, and heats the working fluid to a vapor phase or superheated vapor phase. The disclosure generally describes using heat exchanger76as an evaporator using the engine exhaust60; however, the evaporator is described in greater detail below with reference toFIGS. 3 and 4. Heat exchanger74in the cycle70may be provided as the evaporator.

The expander84may be a turbine, such as a centrifugal or axial flow turbine, or another similar device. The expander84is rotated or actuated by the working fluid to produce work as the working fluid expands. The expander84may be connected to a motor/generator86to rotate the motor/generator to generate electrical power, or to another mechanical linkage to provide additional mechanical power to the driveshaft and wheels55. The expander84may be connected to the generator86by a shaft or another mechanical linkage. The generator86is connected to the battery58to provide electrical power to charge the battery58. An inverter or AC-DC converter88may be provided between the generator86and the battery58.

The working fluid leaves the expander84and flows to a heat exchanger90, also referred to as a condenser90in the cycle70. The condenser90may be positioned in a front region of the vehicle10. The condenser90is configured to be in contact with an ambient air flow92such that heat is transferred from the working fluid to the ambient air flow92to remove heat from the working fluid and cool and/or condense the working fluid. The condenser90may be single stage or multiple stages, and the flow of the working fluid may be controllable through the various stages as required by the cycle70using valves or other mechanisms.

In some examples, the cycle70includes a fluid accumulator94or dryer. The accumulator94may be provided as a fluid or liquid reservoir for the working fluid in the cycle70. The pump72draws fluid from the accumulator94to complete the cycle70. As can be seen fromFIG. 2, the cycle70is a closed loop cycle such that the working fluid does not mix with other fluids in the vehicle or with ambient air.

The cycle70may include a controller96that is configured to operate the cycle within predetermined parameters as described below. The controller96may be in communication with the pump72, expander84, and various valves and/or sensors in the cycle70and vehicle10.

The controller96may be incorporated with or be in communication with an engine control unit (ECU), a transmission control unit (TCU), a vehicle system controller (VSC), or the like, and may also be in communication with various vehicle sensors. The control system for the vehicle10may include any number of controllers, and may be integrated into a single controller, or have various modules. Some or all of the controllers may be connected by a controller area network (CAN) or other system. The controller96and the vehicle control system may include a microprocessor or central processing unit (CPU) in communication with various types of computer readable storage devices or media. Computer readable storage devices or media may include volatile and nonvolatile storage in read-only memory (ROM), random-access memory (RAM), and keep-alive memory (KAM), for example. KAM is a persistent or non-volatile memory that may be used to store various operating variables while the CPU is powered down. Computer readable storage devices or media may be implemented using any of a number of known memory devices such as PROMs (programmable read-only memory), EPROMs (electrically PROM), EEPROMs (electrically erasable PROM), flash memory, or any other electric, magnetic, optical, or combination memory devices capable of storing data, some of which represent executable instructions, used by the controller in controlling the vehicle or the cycle70.

The vehicle may also be provided with an air conditioning system100in one or more embodiments. The air conditioning system100may form a part of a heating, ventilation, and air conditioning (HVAC) system for the vehicle10. The HVAC100system provides air at a controlled temperature to the vehicle or passenger cabin for cabin climate control by the vehicle occupants. The air conditioning system100has a first heat exchanger101or condenser in contact with the ambient air92. The condenser101may be positioned in the front region of the vehicle10. The condenser101is configured for heat transfer between ambient air and a refrigerant or other fluid in the system100.

The air conditioning system100may also include an expansion device, valve, or throttle102, and a compressor or pumping device104. The system100has another heat exchanger106in contact with air flow110to be directed to the vehicle cabin108, and the refrigerant in the system100. Air flow110, which is intended for cabin conditioning, flows over and is cooled by refrigerant in the heat exchanger106, and then flows to the cabin108as required by the vehicle occupants.

FIG. 2illustrates a pressure-enthalpy chart for the working fluid of the Rankine or thermodynamic cycle70as shown inFIG. 1. The chart has pressure (P) on the vertical axis and enthalpy (h) on the horizontal axis. Enthalpy may have units of energy per unit mass, e.g. kJ/kg.

The dome120provides a separation line between the various phases of the working fluid. The working fluid is a liquid or sub-cooled liquid in region122to the left of the dome120. The working fluid is a vapor or superheated vapor in region126to the right of the dome120. The working fluid is a mixed phase, e.g. a mixture of liquid and vapor phase, in region124underneath the dome120. Along the left hand side of the dome120, where region122and124meet, the working fluid is a saturated liquid. Along the right hand side of the dome120, where region124and126meet, the working fluid is a saturated vapor.

The Rankine cycle70ofFIG. 1is illustrated on the chart according to an embodiment. The charted cycle70is simplified for the purposes of this disclosure, and any losses in the cycle70or system are not illustrated although they may be present in actual applications. Losses may include pumping losses, pipe losses, pressure and friction losses, heat loss through various components, and other irreversibilities in the system. The operation of the cycle70as shown inFIG. 2in simplified to assume constant pressure, and adiabatic, reversible, and/or isentropic process steps as appropriate and as described below; however, one of ordinary skill in the art would recognize that the cycle70may vary from these assumptions in a real-world application. The cycle is charted as operating between a high pressure, PH, and a low pressure, PL. Constant temperature lines are shown on the chart as well, e.g. THand TL.

The cycle70begins at point130where the working fluid enters the pump72. The working fluid is a liquid at130, and may be sub-cooled to a temperature of 2-3 degrees Celsius or more below the saturation temperature at PL. The working fluid leaves the pump72at point132at a higher pressure, PH, and in a liquid phase. In the example shown, the pumping process from130to132is modeled as being isentropic, or adiabatic and reversible.

The working fluid enters one or more heat exchangers at132, for example, heat exchangers74,76. The working fluid is heated within the heat exchangers74,76using waste heat from fluids in one or more vehicle systems. In the example shown, the working fluid is heated using engine coolant and exhaust gas. The working fluid leaves the heat exchangers at point134. The heating process from132to134is modeled as a constant pressure process. As can be seen from the Figure, the process from132to134occurs at PH, and the temperature increases to THat134. The working fluid begins in a liquid phase at132and leaves the heat exchangers74,76in a superheated vapor phase at134. In the example shown, the working fluid enters heat exchanger76as a mixed liquid-vapor phase fluid, and leaves the heat exchanger76in the vapor phase.

The working fluid enters an expander84, such as a turbine, at point134as a superheated vapor. The working fluid drives or rotates the expander as it expands to produce work. The working fluid exits the expander84at point136at a pressure, PL. The working fluid may be a superheated vapor at136, as shown. In other examples, the working fluid may be a saturated vapor or may be mixed phase and in region124after exiting the expander84. In a further example, the working fluid is within a few degrees Celsius of the saturated vapor line on the right hand side of dome120. In the example shown, the expansion process from134to136is modeled as isentropic, or adiabatic and reversible. The expander84causes a pressure drop and a corresponding temperature drop across the device as the working fluid expands.

The working fluid enters one or more heat exchangers at136, for example, heat exchanger90. The working fluid is cooled within the heat exchanger90using ambient air received through the frontal region of the vehicle. The working fluid leaves the heat exchanger90at point130, and then flows to the pump72. An accumulator may also be included in the cycle70. The cooling process from136to130is modeled as a constant pressure process. As can be seen from the Figure, the process from136to130occurs at PL. The temperature of the working fluid may decrease within the heat exchanger90. The working fluid begins as a superheated vapor or vapor-liquid mixed phase at136and leaves the heat exchanger90as a liquid at130.

In one example, the cycle70is configured to operate with a pressure ratio of PHto PLof approximately 3, or in a further example, with a pressure ratio of approximately 2.7. In other examples, the pressure ratio may be higher or lower. The cycle70may be adapted to operate in various ambient environments as required by the vehicle10and its surrounding environment. In one example, the cycle70is configured to operate across a range of possible ambient temperatures. The ambient temperature may provide a limit to the amount of cooling available for the working fluid in the heat exchanger90. In one example, the cycle70may be operated between an ambient or environmental temperature of −25 degrees Celsius and 40 degrees Celsius. In other examples, the cycle70may operate at higher and/or lower ambient temperatures.

The power provided by the cycle70may be a function of the mass flow rate of the waste heat fluid, the temperature of the waste heat fluid, the temperature of the working fluid at point134, and the mass flow rate of ambient air. For example, with exhaust gas providing the sole source of waste heat, the power provided by the cycle70is a function of the mass flow rate of exhaust gas through the heat exchanger76, the temperature of the exhaust gas entering heat exchanger76, the temperature of the working fluid at point134, and the mass flow rate of ambient air. For systems with more than one waste heat source, the mass flow rates and temperatures of each source would also be included for the power provided by the cycle70. In one example, the power out of the cycle70is on the order of 0.5-1.5 kW, and in a further example, is approximately 1 kW for a cycle with exhaust temperatures ranging from 500-800 degrees Celsius, and an exhaust gas mass flow rate ranging from 50-125 kg/hr.

The efficiency of the cycle70with respect to the vehicle10may be determined based on the electric power produced by the generator86, and a rate of heat transfer available from the waste heat sources, e.g. engine exhaust, engine coolant, etc. The rate of heat transfer available is a function of the mass flow rate of the waste heat fluid through the associated cycle heat exchanger and the temperature difference of the waste heat fluid across the heat exchangers. In one example, the cycle efficiency was measured to be above 5% on average using exhaust gas heat only, and in a further example, the cycle efficiency was measured to be above 8% on average for a cycle using exhaust gas waste heat only.

Maintaining the state or phase of the working fluid at specific operation points within the cycle70may be critical for system operation and maintaining system efficiency. For example, one or both of the heat exchangers74,76may need to be designed for use with a liquid phase, a mixed phase fluid, and a vapor phase fluid. The working fluid may need to be a liquid phase at point130in the cycle to prevent air lock within the pump72. Additionally, it may be desirable to maintain the working fluid as a vapor between points134and136based on the expander84construction, as a mixed phase may reduce system efficiencies or cause wear on the device84. Based on the ambient air temperature, and the speed of the vehicle, which controls the ambient air flow rate, the amount and/or rate of cooling that is available to the working fluid within the heat exchanger90may also be limited. Furthermore, the amount and/or rate of heat available to heat the working fluid may be limited at vehicle start up when the engine exhaust and/or engine coolant has not reached their operating temperatures.

The cycle70may be operated at various operating conditions, for example, based on a minimum ambient air operating temperature, TL,minand a maximum ambient air operating temperature, TH,max. The working fluid is selected based the cycles and operating states of the various points in the cycle, and the constraints imposed by these operating states. Additionally, the cycle70may be controlled to operate within a desired temperature and pressure range by modifying the flow rate of exhaust gas or other waste heat source through the heat exchangers74,76, thereby controlling the amount of heat transferred to the working fluid and its temperature at point134. The heat exchanger90may also be controlled by providing additional stages, or limiting stages for working fluid to flow through based on the ambient air temperature, flow rate, and humidity, thereby controlling the amount of cooling and the working fluid temperature at point130. Additionally, the flow rate of the working fluid may be controlled by the pump72, such that the working fluid has a longer or shorter residence time in each heat exchanger74,76,90, thereby controlling the amount of heat transferred to or from the working fluid.

FIG. 3illustrates a heat exchanger150for use with the Rankine cycle70or a similar mixed phase thermodynamic cycle for waste heat recovery in a vehicle. The heat exchanger150is configured as an evaporator for the cycle70. The heat exchanger150may be used as heat exchanger76in the cycle70and is configured to transfer heat between exhaust gases and the working fluid in the cycle70to heat the working fluid. In other examples, the heat exchanger150may be used to transfer heat between another waste heat fluid stream and the working fluid, for example, an EGR gas flow.

The heat exchanger150has a housing152surrounding a series of heat exchanger tubes154or chambers. The heat exchanger150may have one heat exchanger tube154, or may have two, three, five, ten, or any number of tubes154or chambers. An inlet manifold156provides a flow of working fluid in the thermodynamic cycle70to the heat exchanger150. The inlet manifold156is connected to an inlet header158. The inlet header158has a series of tubes each connected to and providing liquid phase working fluid to an associated heat exchanger tube154. An outlet manifold160has an outlet header162with tubes connected to associated heat exchanger tubes154. The outlet manifold160and outlet header tubes162receive the vapor phase working fluid from the heat exchanger tubes154such that the working fluid continues to flow through the thermodynamic cycle.

The heat exchanger150has a longitudinal axis170, a transverse axis172, and a vertical axis174. The heat exchanger tubes154are illustrated having a longitudinal axis that is generally parallel with the longitudinal axis170, e.g., the heat exchanger tubes154extend generally parallel with the longitudinal axis170. The vertical axis174may be generally aligned with the gravitational force on the heat exchanger150. The longitudinal axis170and transverse axis172may be generally perpendicular to the vertical axis174such that they both lie in a horizontal plane of the heat exchanger150. As the heat exchanger150may be used in a vehicle10with a cycle70as described above, the axes170,172,174may deviate from true vertical and horizontal as the vehicle10moves over various grades. However, the vertical axis174retains at least a component of a vertical gravitational force as the vehicle travels over various grades.

The inlet manifold156is positioned in a thermodynamic cycle such as cycle70to be downstream of a pump or the like. The inlet manifold156receives working fluid in a liquid phase or mixed liquid vapor phase. In other examples, the working fluid may be a vapor phase, for example, when another heat exchanger is positioned between the pump and the heat exchanger150in the cycle. The working fluid containing a liquid phase flows through the inlet manifold tube156. Although only one inlet manifold tube156is shown, the heat exchanger150may also have additional manifold tubes, valves controlling fluid flow, and the like in other examples. The inlet manifold156may extend in the transverse direction and be generally parallel with the transverse axis172. In other examples, the manifold tube156may be positioned otherwise in the heat exchanger150. The inlet manifold156may provide for generally horizontal flow of the working fluid therethrough.

The inlet manifold156has an inlet header158including one or more inlet header tubes or inlet risers180to direct the working fluid to the various heat exchanger tubes154. The inlet header158may include one or more inlet risers180for each heat exchanger tube154. The inlet risers180fluidly connect the inlet manifold156to the heat exchanger tubes154. Each inlet riser180may contain a section182providing a vertical flow component for the working fluid. As shown inFIG. 3, the risers180have a first section connected to the manifold156and a generally vertical section182connected to the tube154. The first section and section182may be perpendicular to one another or arranged at another angle relative to one another. In other examples, the risers180may have another shape, or may only have a straight section, such as section182connecting the manifold156to the tube154. As the tubes154are arranged in an array, the various tubes180of the header158may vary from one another to connect the manifold156to the heat exchanger tubes154.

The heat exchanger tubes154are provided in the heat exchanger150and provide the primary mechanism for heat transfer between the waste heat fluid and the working fluid in the heat exchanger150. The heat exchanger tubes154may extend generally horizontally within the heat exchanger150, and may be generally parallel with the longitudinal axis170as shown. In other examples, the tubes154may be otherwise positioned within the heat exchanger150. The heat exchanger tubes154may be arranged in an array as shown, where they are spaced apart from one another to allow for waste heat fluid to flow therebetween. The spacing and positioning of the heat exchanger tubes154may also provide for control over the flow path of the waste heat fluid through the heat exchanger150. For example, by offsetting adjacent rows of tubes154, turbulent flow may be induced in the waste heat fluid, leading to increased heat transfer.

The heat exchanger tubes154are illustrated as being generally straight tubes. In other examples, the tubes may be curved or otherwise shaped. The tubes154have a first end region184and a second end region186. An intermediate region188is positioned between the two end regions184,186. In one example, as shown, the inlet header158is connected to the first end region184.

The heat exchanger tubes154may have a shell construction, as shown by arrow190. The shell construction for the tubes154provides for flow of the waste heat fluid over an inner wall and an outer wall of the tube154, thereby increasing the surface area of the tube154and increasing the heat transferred from the waste heat fluid to the working fluid. In another example, the tubes154are provided as standard tubes without a shell construction. In further examples, the tubes154may have multiple layers of a shell style construction providing additional surface area for heat transfer.

The outlet manifold160has an outlet header162including one or more outlet collection tubes200that receive the working fluid from the various heat exchanger tubes154. Each collection tube200of the outlet header162may include one or more outlet risers202. The outlet risers202and collection tube200fluidly connect the heat exchanger tubes154to the main outlet tube of the outlet manifold160.

Each outlet riser202may contain a section providing a vertical flow component for the working fluid. As shown inFIG. 3, the risers202have a generally vertical section connected to the tube154. The risers202provide an exit for a vapor phase of the working fluid at multiple locations from the heat exchanger tubes154. In the example shown, a plurality of risers202are provided for each heat exchanger tube154, with one riser202connected to the tube154at the first end region184, another riser202connected to the tube154at the second end region186, and additional risers202connected to the tube154across the intermediate region188. The risers202may be connected to the tube154and spaced apart along a longitudinal axis204of the tube154. The risers202may be equally spaced from one another, or there may be variable spacing between the risers202. The risers202may have the same cross sectional area, or may have varying cross sectional areas to provide additional control over the flow of the working fluid. As the tubes154are arranged in an array, the various tubes of the header162including the risers202may vary from one another to connect the manifold160to the heat exchanger tubes154.

The collection tubes200are each positioned above a respective heat exchanger tube154and may be generally parallel to the longitudinal axis204. The collection tube200may generally extend the length of the heat exchanger tube154, and fluidly connects the risers202with the outlet manifold160tube. In one example, the collection tube200and the risers202are positioned generally perpendicular to one another.

The primary tube of the outlet manifold160is positioned in a thermodynamic cycle to be upstream of an expander, or the like. The outlet manifold160provides working fluid in a vapor phase or superheated vapor phase. Although only one outlet manifold tube160is shown, the heat exchanger150may also have additional manifold tubes, valves controlling fluid flow, and the like in other examples. The outlet manifold160may extend in the transverse direction and be generally parallel with the transverse axis172. In other examples, the manifold tube160may be positioned otherwise in the heat exchanger150. The outlet manifold160may provide for generally horizontal flow of the working fluid therethrough. The outlet manifold160may be opposed to the inlet manifold156such that the heat exchanger tubes154are positioned between them. In other examples, the inlet and outlet manifolds156,160may be on the same side of the heat exchanger150and adjacent to one another.

The heat exchanger tubes154may be supported by the housing152, for example, at the ends of the housing. The housing is provided with an inlet port206and an outlet port207for the waste heat fluid. In the example shown, the inlet port206is provided on one end plate of the housing152, and the outlet port207is provided on the other end plate of the housing152. The inlet and outlet ports206,207may be connected to an exhaust gas system for an engine, or another vehicle system providing waste heat for use in the Rankine or thermodynamic cycle. The heat exchanger150as shown is configured as a counterflow heat exchanger where the working fluid and the waste heat fluid travel in opposed directions. In other examples, the heat exchanger150may be configured as a parallel flow heat exchanger, a cross flow heat exchanger, or the like. The heat exchanger150may be a once-through heat exchanger where the working fluid only makes a single pass through the heat exchanger and does not cycle or recirculate within it.

The housing152may be provided with baffles208. The baffles208may provide structural support for the heat exchanger tubes154, the collection tubes200, as well as the outer wall of the housing. The baffles208may additionally support or form a part of the risers202. The baffles208may include various openings209to allow waste heat fluids to pass through. The spacing and positioning of the baffles208may be used to control the flow of the waste heat fluid through the heat exchanger150. Additionally, the openings209in the baffles208may be positioned and sized to control the flow of the waste heat fluid through the heat exchanger150.

The various tubes of the heat exchanger150are illustrated as having a circular cross section; however, other shapes are also contemplated for the tubes of the heat exchanger150, and the various tubes may be the same shape and sizes, or may have different shapes or sizes from one another.

The heat exchanger150may be made from various materials and manufactured accordingly. In the example shown, the heat exchanger150is made from a metal, such as aluminum, and is welded or otherwise connected together. In other examples, the heat exchanger150may be made from other materials based on their thermal conductivities, melting temperatures, and other material properties such as corrosion or chemical resistance, etc. For example, if the waste heat fluid is an engine exhaust gas, the heat exchanger150is configured for operation in a high temperature, for example, with approximately 800 degree Celsius gases. The heat exchanger150may also need to be designed with pressure drops as a consideration, both for the working fluid and for the waste fluid. For example, when engine exhaust gas is used as the waste fluid, the heat exchanger150may be configured to provide a low pressure drop for the exhaust gas across the heat exchanger150to limit a back pressure on the engine.

FIG. 4illustrates a partial cross-sectional schematic of the heat exchanger150to describe the operation of the heat exchanger150, for example, as heat exchanger76in cycle70. The heat exchanger150may be provided as an evaporator for the working fluid in the cycle70.

The working fluid enters the heat exchanger150at the inlet manifold tube156. The working fluid in tube156may be a sub-cooled liquid, a saturated liquid, or a liquid vapor mixed phase fluid. In one example, the working fluid in tube156is at point132on the diagram inFIG. 2as a sub-cooled liquid. In another example, the working fluid may be at another state in region122, region124, or along the left hand side of the dome120. In a further example, the heat exchanger150may be used as a superheater where the working fluid is in a vapor phase within the inlet manifold156. For the purposes of this disclosure, the operation of the heat exchanger150is described as being an evaporator with the working fluid in the inlet manifold156as a sub-cooled liquid, as shown as point132inFIG. 2. The working fluid is heated within the heat exchanger such that the working fluid is a vapor phase or superheated vapor at the outlet manifold160tube, as shown as point134inFIG. 2. Therefore, the heat exchanger150is described as providing the132to134process portion of the cycle70. In other examples with additional heat exchangers in a cycle, the heat exchanger150provides only a portion of the heating between points132and134.

The liquid phase working fluid flows from the inlet manifold156to the inlet headers158. The inlet header180has a vertical section182. The vertical section182is illustrated in FIG.4as being connected to a lower surface210of the heat exchanger tube154and is connected to the outer wall212of the heat exchanger tube154. As can be seen inFIG. 4, the inlet header158is connected at an end region184of the heat exchanger tube154. The tube154has a shell construction.

The inlet header158is connected to the lower surface210to provide a bottom filling function for the heat exchanger tube154. The inlet header158may act as a sump at a low point in the heat exchanger tube154to supply liquid working fluid for evaporation. The inlet header158is positioned at the lower surface210based on the forces on the liquid due to gravity and its higher density than the vapor phase. At least a portion of the gravitational forces are along the vertical axis174. The liquid phase working fluid may fill a portion of the heat exchanger tube154as shown by a liquid level218.

The heat exchanger tube has the outer wall212. The heat exchanger tube154may also have a shell construction as described previously with an inner wall214. The inner and outer walls214,212contain the working fluid within a channel defined by the walls. The waste heat fluid216, for example, an exhaust gas from an internal combustion engine, flows over the inner and outer walls214,212. The inner and outer walls214,212may be circumferentially and concentrically arranged about the longitudinal axis204of the heat exchanger tube154.

The heat exchanger150is illustrated as a counterflow heat exchanger. The waste heat fluid216is at a higher temperature than the working fluid. The waste heat fluid216transfers heat or energy to the working fluid within the heat exchanger tube154. The heat transfer occurs based on both a convective heat transfer process and a conductive heat transfer process. Radioactive heat transfer may also occur. The heat transfer occurs from the waste heat fluid216, across the heat exchanger tube154, and to the working fluid.

As the working fluid is heated within the heat exchanger tube154, the energy or enthalpy of the working fluid increases. As this is a generally constant pressure process, the heat transferred to the working fluid causes a phase change in the working fluid as the latent heat of vaporization for the working fluid is reached. The working fluid transitions from a liquid, to a liquid-vapor saturated mixture, and to a vapor phase at220. The heat exchanger tube154may be positioned generally horizontally or aligned with axis170to provide increased efficiency of the evaporator150and the cycle70. In one example, the heat exchanger150has approximately 90% efficiency with a waste heat fluid supplied to the heat exchanger150at approximately 700 degrees Celsius.

The heat exchanger tube154and risers202allow for direct and immediate phase separation of the working fluid as it evaporates, and a more even temperature distribution within the heat exchanger150. The liquid phase218remains in the heat exchanger tube154, and continues to receive heat from the waste heat fluid216. As the tube is arranged horizontally, the liquid phase has a large contact area with the inner and outer walls for improved heat transfer. Additionally, the liquid phase of the working fluid has a greater free surface with the tube154for evaporation. Due to the geometry of the heat exchanger150and the multiple risers202, the vapor phase has an immediate pathway to flow to the outlet manifold160after evaporating, thereby reducing or eliminating convoluted flow paths, vapor locks, or other regions causing a “hot spot” due to a portion of the vapor phase being trapped by surrounding liquid in a flow channel with continued heating. Generally, the thermal conductivity of a gas phase is significantly lower than that of a liquid phase for the working fluid. For example, liquid phase and gas phase R-134a has a thermal conductivity of 0.092 Watts per meter-Kelvin (W/mK) and 0.012 W/mK, respectively.

The vapor phase220of the working fluid rises in the heat exchanger tube154, and exits the heat exchanger tube154through the risers202. The risers202are spaced apart along the length of the heat exchanger tube154to provide multiple exit ports for the vapor phase. The risers202also are positioned for generally vertical flow of the vapor phase working fluid220. The risers202are connected to an upper surface222of the heat exchanger tube154. The upper surface is generally opposed to the lower surface210. A riser202may be positioned adjacent to each end region184,186of the tube154, and additional risers202may be provided in an intermediate region188of the tube154. Each riser202is shown as extending along a corresponding axis224generally perpendicular to the longitudinal axis204, and in some examples, intersecting the axis204.

The vapor phase working fluid220flows from the risers202into the collection tube200and to the outlet manifold tube160. As shown inFIG. 4, the collection tube200may be generally parallel to the heat exchanger tube154, and spaced apart from the tube154. The outlet manifold tube160is upstream of an expander84in the cycle70.

As can be seen inFIG. 4, the heat exchanger tube154is positioned between the inlet header158and outlet headers202, and is positioned between the inlet manifold156and outlet manifold160.

The controller96as shown inFIG. 1may be used to control the cycle70and the closed loop such that the working fluid is a liquid phase at an inlet to the pump72and a vapor phase at an inlet to the expander84. The controller96may be configured to control the closed loop or cycle such that the working fluid comprises a vapor phase in the plurality of risers202and the working fluid comprises a liquid phase in the inlet header180.

For example, a conventional evaporator provides for a flow of working fluid through enclosed channels where the working fluid absorbs heat from the heat flow and is evaporated into gas. The vapor phase working fluid has a reduced thermal transfer efficiency due to the low thermal conductivity of the vapor compared to a liquid phase. Conventional evaporators include a flow path or heat exchanger chamber for the working fluid that travels up and down within the steamer, for example, following a sine curve for a round steamer. As the density of vapor is lower than that of the liquid, the liquid stays at the bottom of the channel and the vapor moves to the top, creating a “hot spot”, which may lead to thermal fatigue of the evaporator and potential for leak issues.

Various examples of the present disclosure have associated, non-limiting advantages. For example, a heat exchanger for a Rankine or other thermodynamic cycle in a vehicle is provided. The heat exchanger has heat exchanger tubes or chambers for evaporation of a working fluid in the cycle using a waste heat fluid, such as an exhaust gas flowing around the heat exchanger tubes. As the working fluid is evaporated in the heat exchanger tubes, the vapor phase of the working fluid separates from the liquid phase and rises in vertical outlet risers of the outlet header. The liquid phase of the working fluid remains in the heat exchanger tubes and continues to be heated by the waste heat fluid. The remaining liquid in the heat exchanger tubes has a high thermal conductivity and high thermal transfer efficiency compared to the vapor phase. The design of the heat exchanger results in the liquid chambers and gas pipes having a generally even temperature distribution since phases of the working fluid separate as they evaporate.