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Title:
GAS TURBINE ENGINE WITH A SPLIT RECUPERATOR USING A HIGH DENSITY WORKING FLUID
Document Type and Number:
WIPO Patent Application WO/2021/084389
Kind Code:
A1
Abstract:
A gas turbine engine is provided with a split recuperator system which includes at least two heat exchangers connected by one or more ducts through which a high density fluid transfers thermal energy. The specific heat capacity and viscosity of the high density fluid vary with pressure over an operational temperature range. At least one of the heat exchangers cools an exhaust fluid from the engine, and at least one of the heat exchangers heats a fluid flowing into a combustor of the engine. The thermal efficiency of at least one of the heat exchangers is controlled by a pressure regulator.

Inventors:
LIOR DAVID (IL)
Application Number:
PCT/IB2020/059940
Publication Date:
May 06, 2021
Filing Date:
October 22, 2020
Export Citation:
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Assignee:
TURBOGEN LTD (IL)
International Classes:
F25B9/00; F01K25/10; F02C7/10
Foreign References:
US20170356340A12017-12-14
US20140096524A12014-04-10
US20150033737A12015-02-05
Attorney, Agent or Firm:
FRIEDMAN, Mark (IL)
Download PDF:
Claims:
CLAIMS

1. A gas turbine engine comprising a split recuperator system, the system comprising: at least two heat exchangers connected by one or more ducts through which a high density fluid transfers thermal energy; the high density fluid having a specific heat capacity and a viscosity which vary with pressure over an operational temperature range; wherein at least one of the heat exchangers cools an exhaust fluid from the engine; at least one of the heat exchangers heats a fluid flowing into a combustor of the engine, and a thermal efficiency of at least one of the heat exchangers is controlled by a pressure regulator which adjusts a pressure of the high density fluid.

2. The system of claim 1 wherein the high density fluid is a supercritical liquid.

3. The system of claim 2 wherein the supercritical liquid is supercritical carbon dioxide.

4. The system of claim 1 wherein the operational temperature range is 500 to 1000 degrees Celsius.

5. The system of claim 1, wherein a pressure of the high density fluid in the one or more ducts is greater than or equal to 50 bars.

6. The gas turbine engine of claim 1 wherein the gas turbine engine is selected from a group consisting of a turbofan engine, a turbo shaft engine, and a turbo propeller engine.

Description:
Gas Turbine Engine with a Split Recuperator Using a High Density Working Fluid

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional patent application serial number 62/926,611, filed October 28, 2019, by the present inventor, which is incorporated by reference in its entirety herein.

FIELD OF THE INVENTION

The invention relates to recuperated gas turbine engines, and specifically to those having a split recuperator.

BACKGROUND

In the design of gas turbines, a recuperator is used to increase thermal efficiency, by recovering energy from the turbine exhaust gas flow. In an aero engine, such as a turbofan, turbo prop, or turbo propeller engine, the use of a recuperator is justified if the reduction in thrust specific fuel consumption (TSFC) more than compensates for the added weight of the recuperator.

In a split recuperator design for a gas turbine engine, the recuperator consists of two (or more) heat exchangers; of which one is located at the turbine outlet and one is located at the last stage compressor outlet or at the combustor inlet. The two heat exchangers are connected by a duct containing a fluid which does not completely evaporate at the maximum turbine outlet temperature. The duct conveys the fluid from the first heat exchanger to the combustor and from the compressor outlet to the second heat exchanger. To reduce pressure losses when the fluid is a gas, the duct must have a large diameter, and this adds considerably to the overall weight of the recuperator and to the overall TSFC in the case of an aero engine.

SUMMARY OF THE INVENTION

The invention is a lightweight, compact split recuperator in which a high density working fluid, such as supercritical carbon dioxide, flows in a duct joining the two heat exchangers. The high density working fluid enables the use of a duct of small diameter and low weight. The flows in the two heat exchangers are controlled by two or more pressure regulators in order to achieve optimal thermal efficiency over a wide range of engine operating conditions, such as those occurring at different flight altitudes and speeds in an aero engine.

According to one aspect of the presently disclosed subject matter, there is provided a gas turbine engine with a split recuperator system which includes at least two heat exchangers connected by one or more ducts through which a high density fluid transfers thermal energy. The high density fluid has a specific heat capacity and a viscosity which vary with pressure over an operational temperature range. At least one of the heat exchangers cools an exhaust fluid from the engine; at least one of the heat exchangers heats a fluid flowing into a combustor of the engine. The thermal efficiency of at least one of the heat exchangers is controlled by a pressure regulator which adjusts a pressure of the high density fluid.

According to some aspects, the high density fluid is a supercritical liquid, such as supercritical carbon dioxide.

According to some aspects, the operational temperature range is 500 to 1000 degrees Celsius.

According to some aspects, the pressure of the high density fluid in the one or more ducts is greater than or equal to 50 bars.

According to some aspects, the gas turbine engine is a turbofan engine, a turbo shaft engine, or a turbo propeller engine.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described herein, by way of example only, with reference to the accompanying drawings, wherein:

Fig. 1: A perspective diagram of a turbofan engine with a split recuperator, according to an exemplary embodiment of the invention.

Fig. 2: A plot of pressure vs. specific enthalpy for carbon dioxide. DETAILED DESCRIPTION

Fig. 1 shows a perspective diagram of a turbofan engine 100 with a split recuperator, according to an exemplary embodiment of the invention. Ducts 120 and 220 and heat exchangers 110 and 210 form a closed pressurized tubular system containing a high density fluid, such as supercritical carbon dioxide. The fluid in duct 120 flows from inlet 120 A to outlet 120B, where it enters into hot heat exchanger 110. The fluid in duct 220 flows from inlet 220A to outlet 220B, where it enters into cold heat exchanger 210. The pressure of the high density fluid is controlled by pressure regulators 130 and 230, inserted into ducts 120 and 220, respectively, as shown. Each pressure regulator is controlled by the maximum temperature at its inlet, so as to optimize the thermal efficiencies of heat exchangers 110 and 210. The optimization depends upon the thermodynamic properties of the highly dense fluid, as explained below.

Fig. 2 shows a plot of pressure P in units of bars vs. specific enthalpy H in units of kilojoules per kilogram (kJ/kg) for carbon dioxide (CO2). Lines of constant temperature (in units of °C) are shown in red and lines of constant specific volume (in units of cubic meters/kg) are shown in green. The CO2 critical point, denoted by "C" in the plot, is at temperature Tc = 31.1 degrees Celsius (°C) and pressure Pc = 73.9 bars. At temperatures and pressures greater than or equal to Tc and Pc respectively, CO2 behaves as a high density supercritical liquid, called "SCO2".

The advantage of using sCCE as a working fluid derives from the fact that it behaves as a dense liquid even at temperatures over 500 °C, and that its specific volume, viscosity, and specific heat capacity can be controlled by varying its pressure over the operational temperature range, which is, for example, 500 °C to 1000 °C.

The recuperator efficiency is optimized as follows. Hot heat exchanger 110 removes heat from the turbine exhaust gas flow with a thermal efficiency which is inversely proportional to the product of fluid mass flow rate G (in units of kg/sec) and specific heat capacity at constant pressure Cp. (Cp is equal to the rate of change of specific enthalpy H with respect to temperature at constant pressure, that is, Cp = (cH/cTT)p . The units of Cp are kJ/kg/°C. ) Conversely, cold heat exchanger 210 heats the air flowing into the combustor of the engine with a thermal efficiency which is proportional to the product of G and Cp.

Thus, to optimize the overall thermal efficiency of the recuperator for a fixed flow rate G, it is necessary a) to decrease the value of Cp in hot heat exchanger 110 and b) to increase the value of Cp in cold heat exchanger 210. A preferred way of doing this is to control pressure, using pressure regulators 130 and 230, as shown in Fig. 1.

As an example, suppose the temperature at hot inlet 120A is 500 °C and the pressure is 150 bars. Referring to segment 310 in Fig. 2, one finds that Cp = 1.11 kJ/kg/°C at a pressure of 50 bars and Cp = 1.22 kJ/kg/°C at a pressure of 150 bars. Thus, by using pressure regulator 130 to decrease pressure from 150 to 50 bars, the value of Cp is reduced by 11% and the efficiency of hot heat exchanger 110 is increased by 11%, in accordance with the recuperator optimization criterion.

Conversely, suppose the temperature at cold inlet 220A is 700 °C and the pressure is 50 bars. By using pressure regulator 230 to re-pressurize the fluid to 150 bars; the value of Cp and the efficiency of cold heat exchanger 210 are increased, again in accordance with the recuperator optimization criterion.

Although the invention has been described in conjunction with specific embodiments, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art. Accordingly, the invention is intended to embrace all such alternatives, modifications, and variations that fall within the spirit and broad scope of this disclosure.