Thermal flux mass flowmeter

Abstract: A mass flowmeter for measuring velocity of a flowing fluid employs a thermal sensor (11). The sensor (11) generates heat when supplied with electrical power. A heat sink (25) is located in the fluid flow a selected distance normal to the line of flow of fluid from the sensor. Electrical power is supplied to the sensor (11) to cause a thermal flux to flow from the sensor (11) to the heat sink (25). The flowing fluid modulates this flux. A circuit (23) measures the temperature of the sensor and computes the velocity of the flowing fluid from that measurement.

Description: This invention relates to a method for operating a thermal fluid flowmeter for measuring the velocity of a flowing fluid. BACKGROUND TO THE INVENTION There are many different types of flowmeters for measuring velocity of a fluid. The term "fluid" as used in this application refers both to liquid and gas flow. One category of flowmeter is known as a "thermal" flowmeter. There are two general types of thermal flowmeters. In one type, a flow pipe is employed with a passage for the fluid flow to be measured. One or more electric heaters are located in the flow or the sensor pipe. The heaters apply heat to the fluid as it flows through the sensor pipe. The temperature is measured at two different points in the sensor pipe. The difference in the temperature between the upstream and downstream point can be correlated to velocity. In the second category of thermal mass flowmeters, a heater/temperature sensor is positioned on a boom and immersed into the flowing fluid stream. A circuit senses the temperature response of the sensor as a function of the mass fluid flow rate. Each of these thermal flowmeters has in common the fact that the rate of heat flow into the fluid from the sensor fluid from the sensor is directly proportional to the mass flow rate of the fluid. The accuracy of these conventional thermal mass flowmeters is limited to a relatively narrow range of flow velocities. With low velocities, accuracy is limited by spurious heat losses due to convection and leakage into the environment. For high velocities, accuracy is limited by the finite thermal resistance of the sensor element or elements. In order to avoid the high velocity range limitation, a common technique is to use a main pipe within which are contained laminar flow elements. These laminar flow elements are arranged to produce a certain pressure drop for the desired range of velocities to be measured. The pressure difference upstream and downstream of the laminar flow elements is proportional to the volume flow rate of the fluid to be measured. A sensor pipe branches off from the main pipe and reenters downstream. This sensor pipe carries heating elements and sensors. A much smaller fluid flow will flow through the sensor pipe than the main pipe. The velocity of the flow in the sensor pipe is measured, it being proportional to the main velocity flowing through the main pipe. Even though this is workable, flow rates outside of the design range can still not be accurately measured without modifying the laminar flow elements. In U.S. Patent 4,517,838, Wachi et al. , May 21, 1985, a heat conducting case is shown. The case has a fine groove in a sensor pipe. Heating means is mounted in the fine groove so as to measure the fluid flow. The small size of the sensor pipe necessitated by such a fine groove restricts the high fluid velocity measurement capability of the flowmeter. DE 3617770 discloses a flowmeter of the above mentioned first type. While it is true that heat generated by sensors (3) and (4) may be absorbed by conduit (7), the response of the configuration shown may not be monotonic over the full range of mass fluid velocities. Similarly it will be seen the European Patent Application number EP-A0048359 describes a centrally heated temperature dependent resistor (1) disposed on a mounting plate (3) parallel to the fluid flow. The resistance is measured and converted into temperature, and the velocity of the fluid flow is derived therefrom. This apparatus does not have a heat sink mounted a pre-determined distance from the sensor perpendicular to the line of flow of the fluid nor is there any reference to the need for the thermal flux to flow perpendicularly to the line of flow of the fluid. PCT Application number WO-A-87/00917 describes a heated temperature sensitive resistor (11) disposed on a substrate (18) positioned substantially parallel to the longitudinal centre line (15) of a bore (14) created by upper (12) and lower (13) mating portions of a thermally conductive housing. An additional temperature sensitive reference resistor 20 is also disposed on substrate (18) in the line of flow with sensor (11), and power is supplied to sensor (11) to maintain a given temperature relationship with the reference sensor (18). However, because the sensors are not thermally isolated from each other, the response of this apparatus is not monotonic over the full range of mass flow velocities. The portions 12 and 13 of the housing act primarily to provide physical protection to the sensitive elements of the sensors and as a heat sink for the other electrical components carried outside the duct. No reference is made to the need for the thermal flux to flux normally to the line of flow of the fluid nor to selection of the gap between sensor 11 and portions 12 and 13. US Patent Specification number 4578996 describes two temperature sensitive resistor wires (18) and (19) positioned in an upstream\downstream relationship parallel to each other and across the direction of flow. A heating coil (17) is placed in close proximity to resistor (18). By measuring the relationship between the resistances of the resistance wires (18) and (19), an output signal corresponding to the amount of gas flow is extracted. However, because heat sinks are not employed, the response of the apparatus is sluggish and elaborate means are disclosed in order to compensate for this deficiency. Furthermore, the heat flux flows parallel to and not normal to the line of fluid flow. We have devised a method for operating a thermal fluid flowmeter which reduces the above problems. In the method of the invention a sensor is placed in the main flow of the fluid and a heat sink is placed a pre-determined distance away from the sensor in a line perpendicular to the line of flow of fluid between the sensor and the heat sink. The sensor is capable of generating heat when supplied with electrical power and the thermal flux generated flows substantially unimpededly from the sensor to the heat sink, which absorbs the heat. This thermal flux is modulated by the flowing fluid. Circuitry measures the temperature rise of the sensor and computes from that measurement the velocity of the flowing fluid. I have found that, by locating the sensor and heat sink in this configuration and selecting the gap between the sensor and the heat sink so that the thermal flux flows between the sensor and heat sink substantially normal to the line of flow of the fluid, surprisingly, the temperature rise of the sensor can be accurately related to the fluid velocity over a wide range of velocities and fluid types. SUMMARY OF THE INVENTION Accordingly, the present invention provides a method for measuring the velocity of a flowing fluid by the use of a device which comprises a heat sensor in contact with the fluid which sensor emits a thermal flux upon the passage of an electrical current through the sensor and computing the temperature increase of the sensor over the ambient temperature of the flowing fluid and computing from the temperature increase and the power supplied the velocity of the flowing fluid, wherein: a. the device comprises a sensor (11, 19) in contact with the flowing fluid and a heat sink (25) also in contact with the flowing fluid and spaced apart from the sensor (11, 19) so as to receive thermal flux from the sensor, the heat sink (25) being located a pre-determined distance from the sensor (11, 19) in a direction substantially normal to the line of flow of the fluid; and in that b. the distance between the sensor and heat sink is chosen so that the thermal flux generated by the sensor (11, 19) flows substantially unimpededly from the sensor (11, 19) to the heat sink (25), which absorbs the thermal flux, and substantially normal to the line of flow of the fluid, said thermal flux being modulated by the flowing fluid. Preferably, the sensor has a resistance which varies with temperature, for example it is a wire coil or wave shaped resistance, and the difference between the resistances at ambient and operating temperatures is used to indicate the temperature rise of the sensor. This is used, with the power applied to the sensor, to calculate the velocity of the fluid as detailed below. It is also preferred that the spaces between the sensor(s) and their co-operating heat sink(s) is substantially unobstructed so that the thermal flux can flow unimpeded between them. The invention also provides a thermal flux meter suitable for use in a preferred embodiment of the invention, which apparatus comprises: a. a reference sensor (37) and an active sensor (35), each adapted to be placed in the flowing fluid, the sensors (37, 35) being capable of generating thermal flux when supplied with electrical power, the sensors (37, 35) being spaced apart from each other in a direction substantially perpendicular to the intended line of the flow of the fluid; b. three heat sinks (39, 41, 43), one of the heat sinks (41) being positioned between the sensors (37, 35), another of the heat sinks (43) being positioned on the opposite side of the reference sensor (37), and the third heat sink (39) being positioned on the opposite side of the active sensor (35), each heat sink (39, 41 and 43) being spaced substantially the same distance from its adjacent sensor (37 or 35) in directions substantially perpendicular to the intended line of flow of the fluid, the distances between the active sensor (35) and its associated heat sinks (41 and 39) being selected so that the thermal flux generated by the active sensor (35) flows substantially unimpededly from the active sensor (35) to the associated heat sinks (41 and 39) and normal to the line of intended flow of the fluid and is modulated by the flow of the fluid; c. means for supplying electrical power to the active sensor (35) to cause a thermal flux to flow from the active sensor (35) to the heat sinks (39 and 41) on each side of the active sensor (35), and for supplying electrical power to the reference sensor (37) at a substantially lower level than the power supplied to the active sensor (35) to cause a thermal flux to flow from the reference sensor (37) to the heat sinks (41 and 43) on each side of the reference sensor (37); and d. means for measuring the temperatures of the active sensor (35) and of the reference sensor (37), for subtracting the temperature of the reference sensor (37) from that of the active sensor (35), means for measuring the power supplied to the active sensor, and means for computing the velocity of the flowing fluid from the temperature difference and the power supplied to the active sensor. DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic representation of a first embodiment of the invention; Figure 2 is a schematic representation of a second embodiment of the invention; Figure 3 is a schematic representation of a third embodiment of the invention; and Figure 4 is a schematic view illustrating in exploded form one of the sensors for use in the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION With reference to Figure 1, a combination heat source/temperature sensor foil 11 is shown. The sensor 11 is mounted in an insulator 13. Sensor 11 is preferably a conventional type element that is normally used to record temperatures. As illustrated in Figure 4, it has a thin insulation layer or substrate 15 of material, for example that sold under the Trade Mark Kapton. The substrate 15 is coated or clad with a thin layer 17 of conductive metal. The metal layer is etched into a sinuous coil pattern 19. Fine very thin lines are cut into the metal layer 17 to create the coil 19. The coil 19 and the substrate 15 are flat surfaces. Sensors of this general type are conventionally available. In this invention, the coil 19 of the sensor 11 is connected to a power source 21, as illustrated in Figure 1, for radiating heat from the sensor 11. Power source 21 provides DC power to flow through the coil 19 (Figure 4) of the sensor 11. The coil 19 of the sensor 11 generates heat, which radiates from the sensor. A measuring circuit 23 measures the power supplied, and thus obtains an indication of the temperature. A heat sink 25 is mounted across from the sensor 11. Heat sink 25 is of a metal that readily conducts heat and thus attracts and absorbs the heat generated from the sensor 11. The surface of the heat sink 25 is flat and parallel with the opposed face of sensor 11. The fluid flow is perpendicular to lines normal to the sensor 11 and heat sink 25 and the space between the opposed faces of the sensor 11 and the heat sink 25 is substantially free from any obstruction. For the purposes of analyzing and describing the behaviour, an incremental volume 27 of the fluid is shown located within an active volume 28 between the sensor 11 and heat sink 25. The following definitions apply: z = distance between sensor 11 and incremental volume 27; dz = thickness of the incremental volume 27; A = area of the incremental volume 27; T o = temperature of the sensor 11; T a = ambient temperature of heat sink 25 and the fluid flowing through the flowmeter; T = temperature of the fluid within the incremental volume 27; Q = heat; C = thermal capacity or specific heat of the fluid (BTU/lb.°F); D = density of the fluid (lb/cu in); K = thermal conductivity of the fluid (BTUin/hr.sq.ft.°F); d = differential operator; t = time; V = average molecular velocity in feet per minute (fpm) of the fluid flowing past sensor 11; and W = power in watts being furnished to the fluid by the sensor 11. A constant fluid velocity profile is assumed across the gap between the sensor 11 and the heat sink 25. The heat Q1 contained in the incremental volume 27 is proportional to the thermal capacity C of the fluid, the mass of the fluid (DAdz), and its temperature T as follows: Q1 = CDTAdz The rate of heat storage in incremental volume 27 is dQ1/Dt minus the rate at which heat is being removed from the element by fluid flowing at velocity V, as follows: dQ1/dt = CDAdz(dT/dt) - CDAdz(T-T a )V = CDAdz[dT/dt - (T-T a ) V] The rate of heat flow or flux from sensor 11 into incremental volume 27 is proportional to the area of the surface A, the fluid conductivity K, and the outward normal gradient of the temperature dT/dz as follows: dQ2/dt = -KAdT/dz The rate of heat flow out of incremental volume 27 is as follows: dQ3/dt = dQ2/dt + d/dz(dQ2/dt)dz = -KAdT/dz - d/dz(KAdT/dz)dz By conservation of heat: dQ2/dt - dQ3/dt = dQ1/dt; -KAdT/dz + KAdT/dz + d/dz(KAdT/dz)dz = CDAdz[dT/dt - (T-T a ) V]; and d²T/dz² = CD[dT/dt - (T-T a ) V]/K In the steady state; dT/dt = 0, and d²T/dz² = CDV(-T + T a )/K This differential equation, along with the following boundary conditions, uniquely describes the thermal environment within the active volume 28 between the sensor 11 and heat sink 25. Hence the steady state equation describing the temperature T for any location z within the active volume 28 is: T = T a + [(G-z)W/KA]eEXP [-(G-z) SQRT (CDV/K)] The steady state equation describing the temperature of the sensor at z = 0 is: T o = T a + [GW/KA]eEXP (-(G) SQRT (CDV/K)] Therefore, the temperature rise TR = T o - T a of the sensor 11 above ambient can be expressed as: TR = (CONSTANT1*G*W/A)*eEXP[-G*SQRT (CONSTANT2*V)], where CONSTANT1 and CONSTANT2 are determined solely by the properties of the fluid. For the units of measurement chosen in this analysis: CONSTANT1 = 482.4/K, and CONSTANT2 = 103,000 (CD/K). This equation has the remarkable property that the overall variation and sensitivity of TR (temperature rise of the sensor 11 over the ambient temperature of the flowing fluid) to fluid velocity can be dictated for any desired fluid type or range of velocities simply by specifying the gap dimension G. In the examples listed below, rather than the preferred etched sensor 11 described above, a prototype sensor was used. It had a coil of sixty turns of 0.0018 inch diameter annealed copper wire sandwiched between two brass disks. Example 1: For P = 0.4 Watts G = 0.025 Inches A = 0.3 Sq. Inches FLUID TYPE = AIR @ STD. TEMP & PRESSURE TR at V = 0 fpm will be 100.5°F; TR at V = 20 fpm will be 74.5°F; and TR at V = 5000 fpm will be 0.8°F. Example 2: For P = 2 Watts G = 0.04 Inches A = 0.3 Sq. Inches FLUID TYPE = WATER TR at V = 0 fpm will be 30.99°F; TR at V = 0.1 fpm will be 21.23°F; and TR at V = 20 fpm will be 0.7°F. Both of these examples illustrate the high resolution available at low flow rates for both air and water, as well as the capability of acquiring measurable data at high flow rates using the present invention. Because the average temperature rise of the fluid in the active volume of the invention at zero velocity is very small compared to conventional thermal flowmeters, posture and convection errors are negligible. The second embodiment of Figure 2 illustrates how to minimize spurious heat losses due to the insulating surface 13 of Figure 1. In Figure 2, the sensor 29 is the same as the sensor 11 of Figure 1, however, it is suspended equidistant between two heat sinks 31, 33. The fluid flows on both side of the sensor 29. The heat sink surfaces 31, 33, are at the same ambient temperature as the temperature of the flowing fluid. The area of both faces or sides of the sensor 29 is used in calculating the temperature/fluid velocity relationship. Because of the thinness of the substrate of the sensor 29, substantially equal amounts of heat will flow in both directions from the sensor 29. Figure 3 illustrates a third embodiment. In this embodiment, one can compensate for the varying temperatures of the fluid entering the thermal flux fluid flowmeter. In this embodiment, sensor 35 is the measuring or active sensor, similar to sensor 11 or sensor 29 of Figures 1 and 2. Sensor 35 is located equidistant between two heat sinks 39, 41. A second sensor 37 is spaced equidistant between the heat sink 41 and another heat sink 43. Sensor 37 is of the same construction as sensor 35, however, it will be a reference sensor. The reference sensor 37 has thermal properties identical to the active sensor 35, but the power employed in making the reference sensor 37 temperature measurement is set to less than one hundredth of that used in the active sensor 35. In this case, DT is the temperature of the active sensor 36 minus the temperature of the reference sensor 37. Referring still to Figure 3, a battery 45 or DC power source has its positive leads connected to one side of the coils of the sensors 35, 37. The active sensor 35 has the other end of its coil connected to a resistor 47. The reference sensor 37 has its other side connected to a resistor 49. The values of the resistors 47 and 49 are selected so that the heat output from the reference sensor 37 is only a fraction, for example about 1/100 the heat output from the active sensor. In the specific example described resistor 49 is a two hundred ohm resistor and resistor 47 is a ten ohm resistor. The opposite sides of the resistors 47, 49 are connected to the negative side of the power source 45. The negative side of the power source 45 is also connected to a terminal C1 of a conventional analog voltage-to-digital data acquisition system or converter 51. The terminal C2 of the A/D converter 51 is connected between resistor 47 and the active sensor 35. The terminal C3 is connected to the positive side of the battery 45. The terminal C4 is connected between resistor 49 and the reference sensor 37. The A/D converter 51 is connected to a conventional computer 53. The A/D converter 51 collects the analog voltages at its terminal C1, C2, C3, and C4 and supplies digital data to the computer 53 for calculating the velocities. In an embodiment of Figure 3, the gaps between heat sinks 39 and 41 and between heat sinks 41 and 43 are selected to be .025 inches. The values of the resistors 47, 49 yield about 0.4 watts to the active sensor 35 and about one hundredth of that value to the reference sensor 37. The following equations are programmed into the computer S3 to yield the desired quantities: Resistance of active sensor 35: R a = 10 (C3-C2)/(C2-C1) ohms Power being delivered to active sensor 35: W = (C3-C2) (C2-C1)/lO watts Resistance of reference sensor 37: R r = 200 (C3-C4)/(C4-C1) ohms Temperature above ambient of active sensor 35: DT a = 458.01 (R a {with power applied} - R a @ ambient temperature)/(R a @ ambient temperature) degrees F Temperature above ambient of reference sensor 37: DT r = 458.01 (R r {with power applied} - R r @ ambient temperature)/(R r @ ambient temperature) degrees F Temperature rise: TR = DT a - DT r degrees F In general, the expression for apparent fluid velocity is: V = (K/lO3OOOCD) [LOG (482.4GW/AKTR)/G]² Using the following values for the thermal properties of air at 70°F and one atmosphere: C = .24 BTU/lb°F D = .0000464 lb/cu in K = .16 BTU. in/hr sq ft °F and the design constants chosen for the realization of the present invention as an anemometer are: A = .4 sq in G = .025 in The specific apparent fluid velocity equation programmed is: V = 2230 [LOG(188W/TR)]² The resulting output velocity is linear to within five percent over inputs spanning a range from two feet per minute to two thousand feet per minute, and the zero stability and posture error for this embodiment of the invention is less than plus or minus 0.5 fpm over an ambient temperature range of 70-110°F. Even better linearity is possible by making further refinements to the preceding algorithms to correct for such factors as the measurable series and shunt thermal impedances of the active sensor 35, as well as deviations from the assumed constant velocity profile of the fluid due to viscosity or other factors. The invention has significant advantages. The thermal mass flowmeter of this invention improves the high and low velocity limitations of conventional mass flowmeters by employing a unique thermal flux modulation technique to enable the accommodation of a wide range of fluid types and velocities. While the invention has been shown in only three of its forms, it should be apparent to those skilled in the art that it is not so limited but is susceptible to various changes without departing from the scope of the invention.

Claims: A method for measuring the velocity of a flowing fluid by the use of a device which comprises a heat sensor in contact with the fluid which sensor emits a thermal flux upon the passage of an electrical current through the sensor and computing the temperature increase of the sensor over the ambient temperature of the flowing fluid and computing from the temperature increase and the power supplied the velocity of the flowing fluid, wherein: a. the device comprises a sensor (11, 19) in contact with the flowing fluid and a heat sink (25) also in contact with the flowing fluid and spaced apart from the sensor (11, 19) so as to receive thermal flux from the sensor, the heat sink (25) being located a pre-determined distance from the sensor (11, 19) in a direction substantially normal to the line of flow of the fluid; and in that b. the distance between the sensor and heat sink is chosen so that the thermal flux generated by the sensor (11, 19) flows substantially unimpededly from the sensor (11, 19) to the heat sink (25), which absorbs the thermal flux, and substantially normal to the line of flow of the fluid, said thermal flux being modulated by the flowing fluid. A method as claimed in claim 1, wherein the space between the heat sink and the sensor is substantially free from obstructions. A method as claimed in either of claims 1 or 2, wherein the sensor (11) and the heat sink (25) have opposing surfaces that are substantially parallel to each other. A method as claimed in any one of the preceding claims, wherein the sensor (11) comprises a resistive element (19), whose plane is substantially parallel to the opposed face of the heat sink (25). A method as claimed in claim 4, wherein the sensor (11) comprises a substrate (15) clad with a metal layer which defines a resistive element (19). A method as claimed in any one of the preceding claims, wherein the device comprises a sensor (29) and a pair of heat sinks (31, 33) positioned substantially equidistantly to either side of the sensor (29) in directions substantially perpendicular to the line of flow of the fluid. A method as claimed in any one of claims 1 to 5, wherein the device comprises: a reference sensor (37) and an active sensor (35), each placed in the flowing fluid, the sensors (37, 35) being capable of generating thermal flux when supplied with electrical power, the sensors (37, 35) being spaced apart from each other in a direction substantially perpendicular to the line of the flow of the fluid; three heat sinks (39, 41, 43), one of the heat sinks (41) being positioned between the sensors (37, 35), another of the heat sinks (43) being positioned on the opposite side of the reference sensor (37), and the third heat sink (39) being positioned on the opposite side of the active sensor (35), each heat sink (39, 41 and 43) being spaced substantially the same distance from its adjacent sensor (37 or 35) in directions substantially perpendicular to the line of flow of the fluid; electrical power is supplied to the active sensor (35) to cause a thermal flux to flow from the active sensor (35) to the heat sinks (39 and 41) on each side of the active sensor (35); electrical power is supplied to the reference sensor (37) at a substantially lower level than the power supplied to the active sensor (35) to cause a thermal flux to flow from the reference sensor (37) to the heat sinks (41 and 43) on each side of the reference sensor (37); the temperatures of the active sensor (35) and of the reference sensor (37) are measured, the temperature of the reference sensor (37) is subtracted from that of the active sensor (35); and the velocity of the flowing fluid is computed from the temperature difference and the power supplied to the active sensor. A thermal flux meter suitable for use in the method as claimed in claim 7, comprising: a. a reference sensor (37) and an active sensor (35), each adapted to be placed in the flowing fluid, the sensors (37, 35) being capable of generating thermal flux when supplied with electrical power, the sensors (37, 35) being spaced apart from each other in a direction substantially perpendicular to the intended line of the flow of the fluid; b. three heat sinks (39, 41, 43), one of the heat sinks (41) being positioned between the sensors (37, 35), another of the heat sinks (43) being positioned on the opposite side of the reference sensor (37), and the third heat sink (39) being positioned on the opposite side of the active sensor (35), each heat sink (39, 41 and 43) being spaced substantially the same distance from its adjacent sensor (37 or 35) in directions substantially perpendicular to the intended line of flow of the fluid, the distances between the active sensor (35) and its associated heat sinks (41 and 39) being selected so that the thermal flux generated by the active sensor (35) flows substantially unimpededly from the active sensor (35) to the associated heat sinks (41 and 39) and normal to the line of intended flow of the fluid and is modulated by the flow of the fluid; c. means for supplying electrical power to the active sensor (35) to cause a thermal flux to flow from the active sensor (35) to the heat sinks (39 and 41) on each side of the active sensor (35), and for supplying electrical power to the reference sensor (37) at a substantially lower level than the power supplied to the active sensor (35) to cause a thermal flux to flow from the reference sensor (37) to the heat sinks (41 and 43) on each side of the reference sensor (37); and d. means for measuring the temperatures of the active sensor (35) and of the reference sensor (37), for subtracting the temperature of the reference sensor (37) from that of the active sensor (35), means for measuring the power supplied to the active sensor, and means for computing the velocity of the flowing fluid from the temperature difference and the power supplied to the active sensor.

Inventors: MICKLER BRIAN ELLIS, MICKLER, BRIAN ELLIS
Applicants: MICKLER BRIAN E, MICKLER, BRIAN ELLIS
Application year: 1989
Main classification: G01F1
IPC codes: G01P5, G01F1