Patent Application
20120312037
This invention relates generally to thermal management, and particularly to vapor cycle systems. In particular, the invention concerns thermal management for an aircraft-based vapor cycle system.
Modern commercial aircraft typically include a number of different heating and cooling systems for the cabin and cargo bay areas, galley facilities, power electronics, and avionics and radar systems. Each of these components has different thermal requirements and power constraints, making overall efficiency an important design criterion.
Most aircraft cooling systems utilize at least one vapor cycle system or VCS unit. The vapor cycle system includes a compressor for compressing the coolant, and a condenser for condensing the compressed fluid, with heat dispersed to different cooling fluid streams. The coolant then flows through an expansion valve to an evaporator, where the fluid expands and cools.
In some configurations, cold VCS fluid is cycled through an AC pack for cooling cabin air. Alternatively, a number of independent cooling loops can be used to cycle specialized coolants to heat loads distributed throughout the aircraft, cooling the coolant by exchanging heat with the vapor cycle system at the evaporator.
In either configuration, weight and efficiency are always at a premium. This makes thermal management an important design consideration, with particular respect to increasing efficiency, reducing the overall weight and size envelope, and maintaining system reliability and service life.
A vapor cycle system comprises a compressor, a de-superheater, a condenser, and an evaporator. The compressor compresses a coolant to form a superheated vapor, and the de-superheater cools the superheated vapor by heat exchange with a cooling fluid flow, forming a reduced temperature vapor. The condenser condenses the reduced temperature vapor by exchanging additional heat with a cooling fluid flow, forming a condensed liquid.
The condensed liquid is expanded then evaporated in the evaporator, absorbing thermal energy and starting the cycle again. The de-superheater is located downstream of the condenser in the cooling fluid flow, so that the cooling fluid temperature is higher at the de-superheater than at the condenser.
Vapor cycle system 12 includes de-superheater 14, compressor 24, condenser 26, economizer 28 and evaporator 30. In one embodiment, vapor cycle system 12 operates on a two-phase coolant or refrigerant fluid such as 1,1,1,2-tetrafluoroethane or R-134a (hydrofluorocarbon HFC-134a). Vapor cycle system 12 is also operable on older refrigerants such as R-12 (chlorofluorocarbon CFC-12) or R-22 (hydrochlorofluorocarbon HCFC-22), but in modern applications “green” fluids are typically used, including R-134a and other HFC, haloalkane and halocarbon-based refrigerant fluids with relatively short environmental lifetimes and reduced potential for ozone depletion.
The VCS loop is driven by compressor 24, which compresses the refrigerant to a superheated phase. The superheated phase is a gaseous or vapor state, at a temperature and pressure above the saturation and condensation points. De-superheater 14 cools the superheated vapor by exchanging heat with the cooling fluid stream, “de-superheating” the fluid to improve efficiency or decrease weight for the vapor/liquid phase transition in condenser 26.
In particular, de-superheater 14 improves the performance of condenser 26 by delivering fluid in a cooler vapor state, as compared to the superheated vapor output of compressor 24. Cooler vapor condenses more quickly, because less heat must be removed to reach the condensation temperature, increasing the efficiency of vapor cycle system 12.
De-superheater 14 also utilizes a higher-temperature (hotter) flow, downstream of condenser 26 and fuel cooler 20, while condenser 26 use a cooler fluid flow, upstream of fuel cooler 20 and de-superheater 14. Because the de-superheat temperature is higher than the condensation temperature, temperature differential ΔT is less at both de-superheater 14 and condenser 26. This reduces the change in entropy and improves the coefficient of performance, as described below.
Refrigerant exits condenser 26 in a two-phase (liquid/vapor) state, at approximately the condensation temperature. Upstream expansion valve 32A provides a minor expansion into economizer 28, further reducing the temperature before entering evaporator 30 through downstream (major) expansion valve 32B.
The refrigerant fluid enters evaporator 30 as a sub-cooled liquid or compressed fluid, or in a two-phase liquid/vapor state. The refrigerant evaporates and expands in evaporator 30 to produce a gas or vapor phase, absorbing heat from cooling system 16 as described below.
Fluid from evaporator 30 enters compressor 24 in a substantially gaseous or vapor state. In two-stage compressor embodiments, as shown in
Cooling system 16 absorbs heat from load 36, and disperses the heat through thermal transfer to evaporator 30 of vapor cycle system 12. In some cooling systems 16, heat is also dispersed to the cooling air flow via heat exchanger (HX) 38.
Heat load 36 includes one or more aircraft systems that require heating or cooling, for example a cabin, cockpit, cargo bay or galley chiller, or electronics components for radar, weapons control, avionics and cabin-based electronics or display systems. In power electronics cooling system (PECS) embodiments, heat load 36 may include power electronics for flight control actuators and other flight-critical systems.
Cooling system (or cooling loop) 16 operates on a refrigerant fluid with high heat transfer capability over a wide temperature range, for example polyalphaolefin (PAO) or hydrogenated PAO. Alternatively, cooling system 16 operates on a silicate ester or oil-based coolant fluid such as Coolanol®, as available from Exxon Mobile of Fairfax, Va., or another fluid such as water, glycol, etc.
The relative flows of cooling system refrigerant through heat exchanger 38 and evaporator 30 are controlled via bypass valves 40A and 40B, based on cooling demands, ambient temperature, flight conditions and the available cooling air flow. For example, bypass valves 40A and 40B can be adjusted to regulate higher levels of air cooling in heat exchanger 38 during cruise flight conditions, and higher levels of evaporator cooling in evaporator 30 during takeoff and landing, or during ground operations.
Fuel circulation system 18 comprises fuel-air cooler 20, fuel tank 42, return-to tank (RTT) cooler 44, fuel-oil cooler (FOC) 46, and secondary (fuel-air) heat exchanger 48. Fuel system 18 typically operates on a kerosene-type jet fuel such as Jet A or Jet A-1, or a naphtha-type fuel such as Jet B for low-temperature performance. In military applications, fuel system 18 operates on a modified kerosene-based fuel such as JP-5 or JP-8, or a modified naphtha or “wide-cut” fuel such as JP-4.
Heat transfer in fuel circulation system 18 is determined according to the temperature requirements of the various fuel subsystems, and based on the different performance demands and fuel system capabilities of military-type aircraft, as compared to commercial designs. In the particular embodiment of
On afterburning turbofan engines, boost pump 50 supplies fuel to inlet valve 52 for afterburner (AB assembly) 54, in order to provide thrust augmentation during short periods of peak operational demand. Boost pump 50 also provides a downstream pressure drop through fuel cooler 20, and generates an overpressure to limit cavity formation at the inlet to main fuel pump 56.
Main fuel pump 56 drives flow through fuel-oil-cooler 46 and secondary heat exchanger 48. Fuel-oil-cooler 46 accepts heat from oil heat load 58, including rotor bearings and other elements of a combustion turbine or turbofan engine (e.g., the main engines for a jet aircraft). Alternatively, oil heat load 58 represents a gearbox or other differential rotation system for a turboprop, turboshaft or geared turbofan engine. Secondary heat-exchanger 48 comprises a fuel-air heat exchanger to cool the compressed air flow from integrated power unit 22, and to pre-heat the fuel before combustion in burner 60.
Valve 62 regulates the recirculation of fuel flow back through RTT cooler 44 to fuel tank 42. As opposed to secondary heat-exchanger 48, which raises the fuel temperature by exchanging heat with compressed air flow from integrated power unit 22, RTT cooler 44 exchanges heat with the expanded cooling air flow to reduces fuel temperatures for storage in fuel tank 42.
Integrated power unit 22 includes an air-cycle machine with compressor 64 and turbine 66, or an auxiliary power unit (APU) comprising compressor 64 and turbine 66 in flow series with a combustor or burner. In some embodiments, generator 68 is rotationally coupled to turbine 64 and compressor 66, for example using a coaxial shaft and clutch mechanism to swap generator 68 in and out during ground operations, or based on flight conditions and real-time electrical demand.
Source 70 of bleed air comprises a compressed air supply such as a first-stage compressor bleed or fan air bleed from the main engine, or a third-stream air source such as an independently modulated bleed flow from a downstream compressor section. Alternatively, bleed air is provided by a ram air intake. In further embodiments, source 70 comprises a static inlet for use during ground operations, or a compressed air supply generated by an APU.
Incoming air is compressed and heated by compressor 64, then cooled by heat transfer to fuel circulation system 18 in secondary (fuel-air) heat exchanger 48. In some embodiments, a primary heat exchanger may also be included, typically upstream of compressor 64.
The compressed air exchanges heat with the downstream cooling air flow in regenerator (air-air heat exchanger) 72, then expands in turbine 66 to produce a low temperature, relatively low-pressure cooling air flow for thermal management system 10. In air-cycle machine embodiments, there is an overall pressure drop from source 70 to the outlet of expansion turbine 66 or energy input from a motor/generator, providing the energy required to turn compressor 64 and generator 68.
Depending on embodiment, cooling air from integrated power unit 22 may be mixed with additional cooling fluid from air source 74. Air source 74 includes an additional fan or compressor bleed air supply, a ram air intake or a third-stream compressed air source providing a supply of relatively cool compressed air. Valve 76 regulates or switches the source between integrated power unit 22 and air source 74, depending on flight conditions, ambient pressure and temperature, and cooling demands.
Downstream of mixer valve 76, the cooling air flow exchanges heat with vapor cycle system 12, cooling system 16 and fuel circulation system 18. Generally, temperatures increase in the downstream direction, as heat is transferred to the cooling air from different components of thermal management system 10. The order of the flow series thus depends on the individual cooling needs of each component, as well as the temperature differential and corresponding entropy and efficiency considerations.
There is an advantage in using the hottest available sink of thermal energy, as compared to the source temperature; that is, with small temperature differential ΔT, because this allows more heat to be rejected from the system. To promote rapid heat transfer, however, larger differentials are desired, because the heat transfer (Q) is proportional to temperature differential ΔT. That is,
Q=hAΔT, [1]
where h is the heat transfer coefficient and A is the heat transfer surface area.
Thermal management also depends on other critical design factors including condensation points and other phase transition temperatures, thermal loading, and environmental (ambient) temperatures and pressures, as compared to the desired cabin and cargo bay conditions across a full range of different flight conditions, and the operating temperature ranges for heat loads including galley chillers, avionics, radar systems and power electronics. Thermal management thus requires constant tradeoffs among different air, fuel, oil, cooling system and VCS components, presenting an almost unlimited number of possible system configurations and corresponding design choices, and making the net results of any particular change or modification difficult to predict.
In the embodiment of
Within vapor cycle system 12, cooling air flows through condenser 26 first, in order to effect a vapor/liquid phase transition at a temperature at or below the condensation point. De-superheater 14 is downstream of condenser 26 in the cooling flow series, so that the de-superheat temperature is above the condensation temperature. The superheated vapor phase is hotter than the condensate, so heat can be transferred at higher temperature (lower ΔT), thereby reserving the cooler air for heat sources that required a lower temperature sink.
As shown in
Depending on embodiment, downstream air may be used to cool thrust nozzle 78, or other main engine components such as blade or vane airfoils for the compressor and turbine section, or hot components of afterburner assembly 54 and burner 60. After core engine or nozzle cooling, cooling air is typically vented to the outside atmosphere.
As shown in
Air-cooled de-superheater 14 include a vapor-air heat exchanger to cool the superheated VCS fluid by exchanging heat with the cooling air flow, lowering the superheated VCS fluid temperature before entering condenser 26. The air temperature is increased downstream of condenser 26, reducing ΔT (and the change in entropy) at de-superheater 14. This increases efficiency and raises the system coefficient of performance, as described above.
De-superheater 14 exchanges thermal energy with fuel circulation system 18 via the cooling air flow through fuel cooler (air-fuel heat exchanger) 20, in flow series between condenser 26 and de-superheater 14. In particular, fuel cooler 20 raises the cooling air temperature above the condensation point, at which condenser 26 operates. De-superheater 14 exchanges additional thermal energy with the fuel flow via downstream components of fuel circulation system 18, as shown
Condenser 26 is also air cooled, and is located between cooling system 16 and fuel cooler 20 in the cooling air flow series. Condenser 26 exchanges additional thermal energy with the cooling air flow through the cycling of VCS refrigerant through economizer 28, evaporator 30, compressor 24 and back to de-superheater 14, where de-superheater 14 is located downstream of fuel cooler 20 in the cooling air flow. Condenser 26 exchanges thermal energy with fuel circulation system 18 via air flow over fuel cooler 20, and via downstream components as described above for de-superheater 14.
Fuel-cooled de-superheater 14 comprises a fuel-vapor heat exchanger to cool the superheated VCS fluid by direct heat exchange with the fuel flow, lowering the superheated refrigerant temperature before entering condenser 26. The fuel temperature is increased downstream of secondary heat exchanger 48, lowering ΔT for de-superheater 14 and reducing the change in entropy to improve the coefficient of performance and overall operating efficiency, as described above.
Condenser 26 is also fuel-cooled, and is located downstream of fuel cooler 20 and upstream of boost pump 50 in the fuel flow series. In this embodiment, condenser 26 condenses the cooled, superheated VCS fluid by direct heat exchange with the fuel flow. Bypass valve 82 regulates the relative fuel flow through fuel tank 42 and fuel cooler 20, maintaining the fuel temperature below the condensation point to encourage a vapor/liquid phase transition in condenser 26.
While this invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the spirit and scope of the invention. In addition, modifications may be made to adapt a particular situation or material to the teachings of the invention, without departing from the essential scope thereof. Therefore, the invention is not limited to the particular embodiments disclosed herein, but includes all embodiments falling within the scope of the appended claims.
