Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
1999-07-01
2001-10-16
Capossela, Ronald (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
C062S637000, C165S060000
Reexamination Certificate
active
06301928
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method for improving the performance of a cryogenic heat exchanger under frosting conditions, and more specifically to a method suitable for a precooler of an air breathing engine.
In order to realize a liquid air cycle engine or a precooled turbojet engine, the development of a high-performance precooler is a key technology. The foremost concern in the development of such a precooler is frost formation on the heat transfer surface of the heat exchanger.
When frost is formed on a heat transfer surface, the efficiency of heat transfer decreases due to the thermal resistance of the frost layer. Further, the flow path of the main air flow narrows, and the pressure loss of the flow increases. The frost layer formed on the heat transfer surface in a cryogenic state has a low density and a low thermal conductivity, and therefore the performance of the heat exchanger is greatly influenced.
In case of the precooler of an air breathing engine used for a space craft, the frost formation creates a trouble particularly when it is flying at low altitude, which is in the acceleration phase. During this period (about several tens to several hundreds of seconds), the operation of the engine cannot be stopped. Therefore, a defrosting cycle, which is applicable in the heat exchanger used for refrigeration or air-conditioning, cannot be applied.
BRIEF SUMMARY OF THE INVENTION
The present invention has been proposed in consideration of the above-described problem, and the object thereof is to provide a method for preventing the decrease of the efficiency of heat transfer, due to frost formation on a heat transfer surface in a cryogenic heat exchanger under frosting conditions, such as a precooler of an air breathing engine.
According to the method of the present invention, into a flow of a gas to be cooled which contains a frost formation material, condensable gas having a melting point lower than that of the frost formation material is mixed at the upstream side of the heat exchanger, and thus the condensable gas is condensed or sublimated together with the frost formation material on the heat transfer surface.
The frost layer thus formed contains a condensed liquid form or sublimated solid form of the condensable gas. Therefore, as compared to the frost layer formed when the frost formation material is condensed solely, this frost layer contains less cavities and has a high density. Therefore, as the cavities decrease, the thermal resistance of the frost layer is reduced. Further, as the thickness of the frost layer decreases in accordance with the reduction of cavities, the flow resistance of the gas to be cooled decreases. Consequently, it becomes possible to prevent the decrease of the performance of heat exchanger, which might be caused by the frost formation.
It should be noted that in order to have the condensable gas condensed or sublimated in about the same amount as that of the frost formation material, it is necessary that the difference in the concentration of saturated vapor of the condensable gas between the temperature of the main flow and that of the heat transfer surface, should be larger than the amount of the frost formation material contained in the main flow. Here, an increase in the necessary amount of the condensable gas thus mixed is not practically advantageous, the concentration of saturated vapor itself should be small preferably. In the case where the cryogenic heat exchanger is a precooler of an air breathing engine, the gas to be cooled is air, and the frost formation material is water vapor. In this case, examples of the condensable gas which satisfies the above conditions, are hydrocarbons of a molecular weight of 90 to 120 (toluene, n-heptane, ethylbenzene, n-octane, isooctane), lower alcohols having 3 or less carbon atoms (methanol, ethanol, propanol), and ethyl acetate.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.
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Harada Kenya
Tanatsugu Nobuhiro
Capossela Ronald
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
The Director-General of the Institute of Space and Astronautical
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