| Flow boiling heat transfer occurs in important parts of various industries such as petroleum and chemical industry,and is also the main heat exchange process in the evaporator of the refrigeration system.The quality of the heat transfer performance of the heat exchanger directly affects the quality of the system performance.Therefore,the heat exchange technology is of great significance to the energy saving and operating stability of the system.This paper starts with convection heat exchange on the refrigerant side,and explores a method to improve the efficiency of refrigerant convection heat exchange by changing the dryness of the refrigerant in the tube.Firstly,the physical parameters in RefProp software are called by MATLAB software.The heat transfer model of an indirect cooling refrigeration system using a tubular evaporator is established with water as refrigerant medium and R134a as refrigerant.The influence of refrigerant convection heat transfer coefficient on the operation of the system is studied by numerical simulation.The following conclusions are drawn:For every 100 W·m-2·?-1increase of refrigerant convective heat transfer coefficient,the heat transfer area decreases by 1.5%to 2.7%when refrigerant convective heat transfer coefficient is 904 W·m-2·?-1 to1300 W·m-2·?-1,by 1.0%to 1.5%when refrigerant convective heat transfer coefficient is1400 W·m-2·?-1 to 2000 W·m-2·?-1,and by 0.6%to 0.7%when refrigerant convective heat transfer coefficient is 2100 W·m-2·?-1 to 3000 W·m-2·?-1;For every 100 W·m-2·?-1 increase of refrigerant convective heat transfer coefficient,when the convective heat transfer coefficient of refrigerant side is in the range of 904 W·m-2·?-1 to 1800 W·m-2·?-1,the convective heat transfer coefficient of refrigerant medium is increased by 0.3%to 1%,when the convective heat transfer coefficient of refrigerant is in the range of 1900 W·m-2·?-1 to 3000 W·m-2·?-1,the convective heat transfer coefficient of refrigerant medium is increased by 0.1%to 0.2%;The mass flow rate of refrigerant medium can be increased by1.1%to 1.9%when the convection heat transfer coefficient of refrigerant is increased by 100W·m-2·?-1,from 904 W·m-2·?-1 to 1300 W·m-2·?-1,from 1400 W·m-2·?-1 to 2000W·m-2·?-1,by 0.5%to 0.9%from 2100 W·m-2·?-1 to 3000 W·m-2·?-1,by 0.2%to0.4%;With the increase of convective heat transfer coefficient of refrigerant,the outlet temperature of refrigerant medium and COP of refrigeration system are decreasing and increasing respectively.In the experiment of flow boiling heat transfer of R134a in horizontal smooth straight tube,the following conclusions were drawn:under the experimental conditions,the local convective heat transfer coefficient of R134a showed a trend of first decreasing and then increasing and then decreasing with the change of dryness.The dryness corresponding to the maximum value of the coefficient is called the critical dryness;the critical dryness increases with the increase of the refrigerant mass flow rate,decreases with the increase of the heat flux density,the evaporation temperature increases,and the critical dryness will also decrease to some extent,under the experimental conditions,the local convective heat transfer coefficient of R134a decreases first,then rises and then falls with the change of dryness,and the dryness corresponding to the maximum value of local convective heat transfer coefficient is called critical dryness;the critical dryness increases with the increase of refrigerant mass flow rate and with the increase of heat flow density,under the experimental conditions,the mass flow of refrigerant increases from 49.68 kg·m-2·s-11 to 247.94 kg·m-2·s-1,the corresponding critical dryness increases from 0.45 to 0.54,and the heat flow density increases from 5.083 kW·m-2 to 14.897 kW·m-2,the corresponding critical dryness decreases from0.51 to 0.42,the evaporation temperature increases from 0?to 10?,and the critical dryness decreases from 0.50 to 0.45;Under the same evaporation temperature and heat flow density,the larger the refrigerant mass flow is,the higher the local convection heat transfer coefficient of R134a corresponding to the same dryness is,and this trend is more obvious in the range of dryness 0.3 to 0.6;Under the same refrigerant mass flow and evaporation temperature,when the dryness is less than 0.6,the higher the heat flux density is,the greater the local convection heat transfer coefficient of R134a corresponding to the same dryness is,when the dryness is greater than 0.6,increasing the heat flux density will lead to the deterioration of flow boiling heat transfer,in addition,it can be found that the higher the heat flux density is,the higher the proportion of nuclear boiling in the whole flow boiling heat transfer is;Under the same condition of refrigerant mass flow and heat flow density,the higher the evaporation temperature is,the larger the local convection heat transfer coefficient of R134a corresponding to the same dryness is.Under different mass flow distribution of R134a,heat flux density and evaporation temperature,the experiment of replenishing R134a was carried out.The replenishing point was at 1/3 of the experimental section.The experimental results showed that:with the heat exchange going on,the local convective heat transfer coefficient of R134a was divided into four stages,the first stage was before the replenishing point,which was similar to the common flow boiling heat transfer,the second stage occurred after the replenishing,About half of the total length of the heat exchange tube,the local convective heat transfer coefficient increases sharply and then decreases rapidly,when the disturbance is gradually stable,it enters the third stage,which accounts for about 1/3 of the length of the heat exchange tube,in this stage,the refrigerant flow pattern in the tube enters the annular flow,with the increase of the dryness,the local convective heat transfer coefficient increases,and in the fourth stage,the local convective heat transfer coefficient decreases;In the same condition,the critical dryness?0.60-0.75?of R134a in the experiment is larger than that in the experiment of horizontal straight tube?0.45-0.58?;In the experiment of this paper,the average convective heat transfer coefficient of R134a in the experiment of replenishing R134a is 15.2-20.3%larger than that in the experiment of horizontal straight tube under the same condition;The distribution of refrigerant mass flow rate in the main flow area and the replenishing part has a great influence on the improvement of heat exchange efficiency. |
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