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1、Joule-Thomson effectThere are five common methods to reduce the temperature of objects:Cooling by objects with lower temperature;Reduce temperature by absorbing latent heat(such as heat of vaporization,heat of adsorption,heat of solution,heat of dilution,etc.);Temperature reduction by adiabatic expa
2、nsion;Thermo-electric cooling;Throttling expansion cooling.Most refrigerators obtain low temperature heat source by liquefying medium gas,and use evaporation and heat absorption of liquid chemicals to provide refrigeration.Throttling effect is an effective means for gaseous working medium to appear
3、as liquid after cooling.It is found that the temperature of the gas on both sides of the porous plug is generally not equal,and the temperature difference is related to the type of gas and the pressure value on both sides of the porous plug.Under adiabatic conditions,the steady flow process of high-
4、pressure gas flowing to the low-pressure side through small holes of porous plugs,valves with small sizes and capillaries is called throttling process.At present,in industry,gas is throttled and expanded through throttle valve or capillary tube.l l Discussion111111VplApW222222VplApW Taking the gas o
5、n the left side of the piston as the research object,when the gas passes through the porous plug,its state parameters are from V1V2,p1p2 ,T1T2.Assume that the internal energy of gas is U1 when it is on the left and U2 when it is on the right Obviously,when the gas passes through the porous plug,the
6、work W1 of the left piston and the work W2 of the gas pushing the right piston are There is a porous plug in the center of the adiabatic cylinder with both ends open,and the two sides maintain different pressures p1,p2,p1 p2 The net work done by the outside world to the quantitative gas is221121VpVp
7、WWW221112VpVpUU21HH WQUNote that the adiabatic process Q=0,then from the first law of thermodynamicsEnthalpy is unchanged before and after adiabatic throttling process.The temperature of the ideal gas is constant before and after throttling process.The ideal gas()UU T()HH T The experiment shows that
8、 for real gas,if the gas type is different and the initial and final pressure is different,the temperature changes will be different before and after throttling process.But for hydrogen and helium,the temperature rises after throttling at normal temperature,which is called negative throttling effect
9、.In low temperature engineering,throttling refrigeration effect is used to reduce temperature.General gases(such as nitrogen,oxygen,air,etc.)all decrease in temperature after throttling at normal temperature,which is called throttling refrigeration effect(or positive throttling effect):In order to s
10、tudy the temperature changes of different kinds of gases under different pressures and temperatures after throttling,various isenthalpic lines are often made in T-p diagram by experiments.The slope of the isenthalpic curve is called Joule-Thomson coefficientHpT)(The ideal gas The real gasDifferent t
11、hrottling effectsi74ipiT4p4TpT The value at the maximum of any isenthalpic line is equal to zero.A curve in which these points are connected is called a transition curve.000Throttle refrigeration zoneThrottling heating zone Conversion zone 00pT HpT)(Enthalpy is unchanged before and after adiabatic throttling process.The temperature of ideal gas is constant before and after throttling process.For the real gas,if the gas type is different and the initial and final pressure is different,the temperature changes will be different before and after throttling.Thanks
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