Abstract:
A regenerative heat exchanger for transferring heat from the exhaust gas to the intake working fluid of a prime mover. Application includes gas turbines for both motor vehicles and distributed electric generation. The heat exchanger employs a rotating matrix, which circulates through working fluid exhaust and intake channels while absorbing and rejecting heat between the two channels. Features include corrugated tubes for enhanced heat transfer, minimally welded low stress construction, quick-detach assembly of standard components, and purge flow sealing using recovered heat. Effectiveness exceeding 95% increases thermal efficiency of low-pressure ratio gas turbines.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to counter flow regenerative heat exchangers for heat recovery in low capacity prime movers. This includes distributed electric generation and vehicle use, and pertains particularly to an improved regenerator for small gas turbine engines. Low capacity gas turbines are generally considered to be impractical, especially in variable speed automotive use, due to very high turbine speed, inefficient turn-down during deceleration and idling, and high exhaust temperature. The rotary regenerator of the present invention with heat transfer enhancing features resolves these issues, enabling efficient low compression operation with effectiveness greater than 95%. Low stress and compact low cost metallic construction withstands high turbine outlet temperature associated with low compression. As a result, cycle efficiency is high within turbine stress limitations imposed by the pressure-speed relation, wherein rotor speed is directly proportional to working fluid flow rate and compression ratio and indirectly proportional to turbine diameter. Additional benefits of enhanced heat transfer in low compression application are reduced leakage of intake air into exhaust gas, improved flow distribution through the heat transfer surface and closer balance between intake air and exhaust gas pressure drop. Estimated cost is 40 $/kW engine capacity 
         [0002]    The regenerator of the present invention employs a rotating matrix of corrugated heat transfer tubes, which absorb heat on both inner and outer surfaces from the lower pressure exhaust gas side for transfer to the pressurized intake air side of the regenerator. Heat transfer in the laminar flow range provides a compact and high effectiveness design. Compactness is further improved using either pre-fabricated honeycomb or packed tubular cell construction in a hexagonal array. Each cell contains one or more corrugated tubes with enhanced heat storage and heat transfer capability. Further compactness is achieved in the tubular type matrix by meshing the ridges and grooves of corrugated tubes in hexagonal groups within the cells. This arrangement provides positive tube positioning in a relatively low stress unconstrained matrix. Heat transfer may be further enhanced in both matrix types by insertion of longitudinal strip-fins within the corrugated tubes. The stainless steel or nickel alloy matrix operates well within recommended service temperature limits approaching normal micro-turbine inlet gas temperature of 1150 K in this low stress application. The honeycomb matrix may be mass produced using a relatively inexpensive automatic welding process and the alternate cell tube matrix is non-welded. The corrugated tubes are readily available and installed without welding at minimal cost. 
         [0003]    The matrix is supported on bearings at each end of a central shaft and rotates through seals having minimal working fluid leakage. Two factors lower seal leakage; low compression ratio of the application and an elongated matrix due to enhanced heat transfer. Matrix length to diameter is reduced from a ratio of about 5 to a ratio of 1 in the regenerator of the present invention. Seal leakage may be further reduced by a purge system, drawing fluid from a turbine bearing air supply or a rotor cooling water supply for distribution along matrix support bars. An electric motor provides rotation of the matrix via a pinion and ring gear. 
         [0004]    Current practice for small gas turbines utilizing heat recovery to increase thermal efficiency is to employ recuperators with fixed surface area for stationary use and rotary regenerators for automotive use. In the former case micro-turbines for distributed electric generation are gaining wide acceptance, while in the latter case gas turbine development is ongoing and limited to constant speed proto-types. The state-of-the-art micro-turbine heat exchanger is a counter-flow recuperator, which operates in the laminar flow range for acceptable heat transfer and effectiveness in a plate type matrix with numerous parallel flow passages fitted with strip-fins. It is the most expensive component of the gas turbine system, constructed of high temperature alloys with a large number of closely spaced brazed joints and complex header arrangements. Efforts are ongoing to develop a less expensive heat exchanger. The state-of-the-art automotive heat exchanger is a more advanced rotary regenerator, which also relies on laminar flow, but in a ceramic disk matrix, It must withstand thermal cycling to nearly turbine inlet gas temperature during deceleration and idling. Both fixed and rotary heat exchangers are subject to design compromise to limit thermal stresses. The fixed metallic recuperator is constrained by thermal expansion and maximum service temperature is limited to about 950 K. Estimated cost is 160 $/kW engine capacity. The ceramic rotary regenerator matrix can withstand elevated turbine exhaust temperature, but off-design operating conditions may impose excessive thermal stress. In addition, the latter is subject to erosion/corrosion due to seal leakage and is not conducive to heat transfer enhancement geometry and ring gear attachment. Estimated cost is 80 $/kW engine capacity 
       SUMMARY AND OBJECTS OF THE INVENTION 
       [0005]    Accordingly, objects and advantages of the rotary regenerator of the present invention are: 
         [0006]    (a) to provide a rotary regenerator for increasing thermodynamic cycle efficiency of expansion engines; 
         [0007]    (b) to provide a rotary regenerator having high effectiveness; 
         [0008]    (c) to provide a rotary regenerator with a low constraint metallic heat transfer matrix to withstand thermal stresses at highest service temperature; 
         [0009]    (d) to provide a rotary regenerator having a compact assembly using a hexagonal matrix; 
         [0010]    (e) to provide a rotary regenerator having low seal leakage or increased pressure capability; 
         [0011]    (f) to provide a rotary regenerator having heat recovering purge flow for matrix lubrication and low seal leakage without loss of engine efficiency; 
         [0012]    (g) to provide a rotary regenerator having uniform matrix flow distribution; 
         [0013]    (h) to provide a rotary regenerator constructed of readily available components including enclosure and heat transfer cells; and 
         [0014]    (i) to provide a rotary regenerator with tube matrix accessibility contained in a quickly detachable enclosure. 
         [0015]    Further objects and advantages are to provide an inexpensive and reliable regenerator, which will enable widespread application of expansion engines including low capacity gas turbines. Still further objects and advantages will become apparent from a consideration of the following description and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    In the drawings, closely related figures have the same number but different alphabetical suffixes. 
           [0017]      FIG. 1A  is a plan view illustrating working fluid channeling and component arrangement of a preferred embodiment of the regenerator of the present invention for prime mover application. Dashed lines depict internal components. 
           [0018]      FIG. 1B  is an end elevation view illustrating working fluid channeling and component arrangement of a preferred embodiment of the regenerator of the present invention. Dashed lines depict internal components. 
           [0019]      FIG. 1C  is a longitudinal cross-section view illustrating component arrangement of a preferred embodiment of the heat transfer matrix of the regenerator of the present invention. 
           [0020]      FIG. 1D  is a transverse cross-section view illustrating component arrangement of a preferred embodiment of the of the heat transfer matrix of the regenerator of the present invention. 
           [0021]      FIG. 1E  is a longitudinal cross-section view of a preferred embodiment of a portion of a corrugated tube of the heat transfer matrix of the regenerator of the present invention. 
           [0022]      FIG. 2A  is an transverse cross-section view illustrating an alternate preferred embodiment of a heat transfer matrix of the regenerator of the present invention. 
           [0023]      FIG. 2B  is a transverse cross-section view illustrating an alternate preferred embodiment of a matrix cell of the regenerator of the present invention. 
           [0024]      FIG. 2C  is a transverse cross-section view illustrating an alternate preferred embodiment of a heat transfer matrix of the regenerator of the present invention. 
           [0025]      FIG. 2D  is a partial longitudinal elevation view illustrating an alternate preferred embodiment of adjacent corrugated heat transfer tubes of the heat transfer matrix of the regenerator of the present invention. 
           [0026]      FIG. 3  is a longitudinal cross-section view illustrating an alternate preferred embodiment of a heat transfer enhancement component of the regenerator of the present invention. 
           [0027]      FIG. 4A  is a schematic illustrating a preferred embodiment of a purge flow system of the regenerator the present invention. Dashed lines depict purge flow distribution. 
           [0028]      FIG. 4B  is an elevation view illustrating a preferred embodiment of a purge flow distribution component of the regenerator the present invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0029]      FIG. 1A  and  FIG. 1B  illustrate working fluid channeling and component arrangement of a preferred embodiment of a rotary regenerator  100  of a prime mover of the present invention. Arrows indicate flow direction of working fluid from a compressor discharge line  102  through a pressurized regenerator channel  104  and discharging to a combustor intake line  106 , while working fluid exhaust from a turbine discharge line  108  continues through a depressurized regenerator channel  110  to atmosphere. Heat is transferred from the turbine exhaust to pressurized working fluid within a rotating heat transfer matrix  112 . The matrix is contained and supported in a containment vessel  114  constructed of two tee fittings  116  held together by a bolted clamp  118 . Clamped stainless steel tee fittings are available from Victaulic Company of Easton, Pennsylvania. Semi-circular baffle plates  120 , welded to the fittings and abutted to a colder matrix end support bar  122  and to a hotter matrix end support bar  124 , divide the pressurized and depressurized channels. Each bar is fitted with shaft bearings  126 , which support a central rotational shaft  128  of the matrix. The matrix is driven by a geared electric motor  130  via a ring gear  132  attached to the matrix. Radial leakage of working fluid across the ends of the matrix is limited by appropriate surfacing of the bars, while a circumferential seal  134  limits longitudinal leakage of working fluid past the matrix and insulation  136  limits heat loss from the vessel. 
         [0030]      FIG. 1C  illustrates component arrangement of a preferred embodiment of the matrix. Longitudinal cells  138  are in a hexagonal honeycomb pattern constructed of longitudinally welded cells to facilitate a low leakage and compact matrix. Honeycomb matrix of stainless steel is available from Benecor, Inc. of Wichita, Kans. The matrix is held in place in the center by shaft  128  and at the periphery by a circular duct  140 .  FIG. 1D  illustrates the pattern of matrix cells.  FIG. 1E  illustrates a corrugated tube  142 , one of which isE inserted in each cell. The corrugated tubes are retained in the cells by retainer plates  144  having an appropriate perforation pattern and flow area greater than flow area through and along the corrugated tubes. Corrugated tubing of longitudinally welded stainless steel is available from Hose Master Inc. of Cleveland, Ohio and in open seam form from George Risk Industries of Kimball, Nebraska. Components in contact with the matrix are shown including the two shaft support bars with bearings, the ring gear and the seal. 
         [0031]    The corrugated heat transfer tubes, with both inside and outside active surfaces, enable low hydraulic diameter of the matrix and high conductive heat transfer coefficient in the laminar flow range. Heat transfer coefficient and friction factor are comparable to that of a fixed plate type recuperator operating in similar flow conditions, but at about one-third of the cost. This is accomplished by elimination of headers and associated welds in conjunction with automated honeycomb matrix production. Performance of the exemplary regenerator is estimated at operating conditions applicable to a compact motor vehicle at a cruising speed of 120 km/h (75 mph). Turbine inlet gas temperature is 1110 K (2000 R) and compression ratio is 3 with exhaust and pressurized side losses limited to 2.5% and 1%, respectively. At these conditions cycle efficiency and regenerator effectiveness are approximately 30% and 92%, respectively. The regenerator is configured as a hexagonal group of 7 corrugated tubes per cell, with sizing based on turbine exhaust temperature of 900 K (1620 R) and heat duty of 460,000 kJ/h (436,000 Btu/h). Heat duty is based on the assumption that a portion of the exhaust is bypassed around the regenerator to avoid surface area penalty during infrequent high power operation. The resulting matrix geometry is; surface area per cell=300 cm 2 (46 in 2 ), flow area per cell=0.65 cm 2 (0.10 in 2 ), total cells=230, hydraulic diameter=0.21 cm (.084 in.), cell and corrugated tube length=30.5 cm (12 in.), and matrix mass per cell=0.045 kg (0.10 lb.). 
         [0032]      FIGS. 2A through 2D  illustrate an alternate preferred embodiment of the matrix  212  of the regenerator of the present invention.  FIG. 2A  is a further cross-section of  FIG. 1C  and illustrates the tubular matrix constructed of cell tubes  238  in a hexagonal pattern. The cell arrangement forms a non-welded matrix held in line contact by compression imposed by a duct  240 . Smaller diameter filler tubes  241  complete fitting of the hexagonal matrix to the circular duct.  FIG. 2B  illustrates a hexagonal group of 7 corrugated tubes  242  inserted in a cell tube.  FIG. 2C  illustrates a perforated retainer plate  244  for holding the corrugated tubes in the matrix. Two plates are held within the duct between the matrix support bars  224 ,  226  and the ends of the cell tubes. Each plate has a flow area through the perforations greater than the flow area through the matrix.  FIG. 2D  is an alternate preferred embodiment  246  of adjacent corrugated tubes illustrating meshing of annular corrugations  248  of the corrugated tubes. Nearly full engagement of the corrugations is expected to decrease the matrix and vessel diameters by about 20% with little effect on hydraulic diameter and heat transfer rate of the matrix. Overall sizing of the tubular cell matrix is comparable to the honeycomb matrix of  FIGS. 1A through 1D , however the corrugated tubes are inserted in hexagonal groups to reduce the number of cell tubes. Performance of the tubular cell matrix is expected to be comparable to the honeycomb matrix. Some additional leakage will occur between cell tubes, however cost is estimated to be 20% as compared to a plate type stationary recuperator operating in similar flow conditions. Cost reduction is accomplished by elimination of headers and welds. The resulting matrix geometry is; surface area per 7 tube cell=(317 in 2 ), flow area per 7 tube cell=4.6 cm 2 (0.72 in 2 ), total cells=43, hydraulic diameter=2.5 cm (0.10 in.), cell and corrugated tube length=30.5 cm (12 in.), and matrix mass per 7 tube cell=0.20 kg (0.45 lb.). 
         [0033]      FIG. 3  is an alternate preferred embodiment illustrating a saw toothed strip fin  346  inserted in a corrugated tube  342 . The strip decreases hydraulic diameter of the tube by an estimated 33% while increasing heat transfer coefficient and friction factor in approximately the same proportion. 
         [0034]      FIG. 4A  is a schematic illustrating a preferred embodiment of a purge flow injection system for limiting working fluid leakage transverse to tube ends and cell ends of the matrix. An exemplary steam purged regenerator  400  is shown in relation to prime mover components including a compressor  450 , a combustor  452  with a fuel tank  454 , and a turbine  456 . Open arrows and dashed lines indicate flow and direction of purge water and steam. Purge flow is from a water tank  458  through a recovery evaporator  460  of the compressor from which a portion of steam is diverted and injected into the tips of a colder end matrix support bar  422 . The remaining portion then continues through a recovery superheater  462  of the combustor for superheating and injection into the tips of a hotter end matrix support bar  424 .  FIG. 4B  illustrates a hollow support bar  422  or  424  connected to water or steam lines of the purge flow supply. Purge flow distribution nozzles  425  are oriented to discharge toward the pressurized channel of the matrix. A shaft bearing  426  is shown oriented at right angles to the steam discharge. 
         [0035]    Working fluid leakage across a non-purged matrix is low because of low compression ratio and high length to diameter ratio of the matrix. The purge system is adaptable in high temperature gas turbines employing a water cooled turbine rotor while reducing surface area of the matrix. This is because of two factors; zero working fluid leakage and enhanced heat transfer with non-luminous water vapor radiation. 
         [0036]    While I have illustrated and described my invention by means of specific embodiments, it is to be understood that numerous changes and modifications may be made therein without departing from the spirit and scope of the invention as defined in the appended claims. For example, prime mover heat input may include solar, the regenerator may be oriented with downward exhaust requiring only one tube retainer plate at the bottom, and fin strips with various cross-section configurations may be inserted in the corrugated tubes.