【導(dǎo)讀】particles.andCelsiusscales.

　　

【正文】 lug in a gasoline engine. 943C The Diesel cycle differs from the Otto cycle in the heat addition process only。 it takes place at constant volume in the Otto cycle, but at constant pressure in the 20 Diesel cycle. 944C The gasoline engine. 945C Diesel engines operate at high pression ratios because the diesel engines do not have the engine knock problem. 101C Because excessive moisture in steam causes erosion on the turbine blades. The highest moisture content allowed is about 10%. 102C The Carnot cycle is not a realistic model for steam power plants because (1) limiting the heat transfer processes to twophase systems to maintain isothermal conditions severely limits the maximum temperature that can be used in the cycle, (2) the turbine will have to handle steam with a high moisture content which causes erosion, and (3) it is not practical to design a pressor that will handle two phases. 107C The four processes that make up the simple ideal cycle are (1) Isentropic pression in a pump, (2) P = constant heat addition in a boiler, (3) Isentropic expansion in a turbine, and (4) P = constant heat rejection in a condenser. 108C Heat rejected decreases。 everything else increases. 109C Heat rejected decreases。 everything else increases. 1010C The pump work remains the same, the moisture content decreases, everything else increases. 1012C The boiler exit pressure will be (a) lower than the boiler inlet pressure in actual cycles, and (b) the same as the boiler inlet pressure in ideal cycles. 1014C Yes, because the saturation temperature of steam at 10 kPa is 176。C, which is much higher than the temperature of the cooling water. 1029C The pump work remains the same, the moisture content decreases, everything else increases. 1039C Moisture content remains the same, everything else decreases. 1040C This is a smart idea because we waste little work potential but we save a lot from the heat input. The extracted steam has little work potential left, and most of its energy would be part of the heat rejected anyway. Therefore, by regeneration, we utilize a considerable amount of heat by sacrificing little work output. 1041C In open feedwater heaters, the two fluids actually mix, but in closed feedwater heaters there is no mixing. 1042C Both cycles would have the same efficiency. 1063C The utilization factor of a cogeneration plant is the ratio of the energy utilized for a useful purpose to the total energy supplied. It could be unity for a plant that does not produce any power. 21 第十一章 111C Because the pression process involves the pression of a liquidvapor mixture which requires a pressor that will handle two phases, and the expansion process involves the expansion of highmoisture content refrigerant. 114C Yes。 the throttling process is an internally irreversible process. 115C To make the ideal vaporpression refrigeration cycle more closely approximate the actual cycle. 116C No. Assuming the water is maintained at 10176。 C in the evaporator, the evaporator pressure will be the saturation pressure corresponding to this pressure, which is kPa. It is not practical to design refrigeration or airconditioning devices that involve such extremely low pressures. 117C Allowing a temperature difference of 10176。 C for effective heat transfer, the condensation temperature of the refrigerant should be 25176。 C. The saturation pressure corresponding to 25176。 C is MPa. Therefore, the remended pressure would be MPa. 118C The area enclosed by the cyclic curve on a Ts diagram represents the work input for the reversed Carnot cycle, but not so for the ideal vaporpression refrigeration cycle. This is because the latter cycle involves an irreversible process for which the process path is not known. 119C The cycle that involves saturated liquid at 30176。 C will have a higher COP because, judging from the Ts diagram, it will require a smaller work input for the same refrigeration capacity. 1110C The minimum temperature that the refrigerant can be cooled to before throttling is the temperature of the sink (the cooling medium) since heat is transferred from the refrigerant to the cooling medium. 1123C The desirable characteristics of a refrigerant are to have an evaporator pressure which is above the atmospheric pressure, and a condenser pressure which corresponds to a saturation temperature above the temperature of the cooling medium. Other desirable characteristics of a refrigerant include being nontoxic, noncorrosive, nonflammable, chemically stable, having a high enthalpy of vaporization (minimizes the mass flow rate) and, of course, being available at low cost. 1124C The minimum pressure that the refrigerant needs to be pressed to is the saturation pressure of the refrigerant at 30176。 C, which is MPa. At lower pressures, the refrigerant will have to condense at temperatures lower than the temperature of the surroundings, which cannot happen. 1125C Allowing a temperature difference of 10176。 C for effective heat transfer, the evaporation temperature of the refrigerant should be 20176。 C. The saturation pressure corresponding to 20176。 C is MPa. Therefore, the remended pressure would be MPa. 1129C A watersource heat pump extracts heat from water instead of air. Watersource heat pumps have higher COPs than the airsource systems because the temperature of water is higher than the temperature of air in winter. 1151C In the ideal gas refrigeration cycle, the heat absorption and the heat rejection processes occur at constant pressure instead of at constant temperature. 22 1152C In aircraft cooling, the atmospheric air is pressed by a pressor, cooled by the surrounding air, and expanded in a turbine. The cool air leaving the turbine is then directly routed to the cabin. 1153C No。 because h = h(T) for idea