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The influencing factors of liquid refrigerant boiling heat transfer

by:Arkool     2020-07-04
In the evaporator, the cooling medium heat is passed to refrigerant heat transfer wall, the liquid refrigerant absorption vaporization. The state of matter changes of refrigerant in the evaporator as evaporation. Inside the evaporator heat transfer effect also like condenser, by the heat transfer coefficient of refrigerant, the thermal resistance of heat transfer surface contaminants, and by the side of the cooling medium to the factors of heat transfer coefficient. The latter two factors are basically the same as the condition of the condenser, the refrigerant liquid heat transfer coefficient and gas condensation heat transfer coefficient of has essential difference. Refrigeration unit within the evaporator heat transfer temperature difference is not big, so the refrigerant liquid boiling in the bubble boiling. The boiling heat transfer surface in many bubbles, the bubbles grow bigger, rising from the surface and in the liquid. After they rise, there is continuous to produce bubbles. Boiling heat transfer coefficient and the size of the bubble, bubble velocity and other factors. Here mainly analyzes the influencing factors of liquid refrigerant boiling heat transfer. The influence of the physical properties of refrigerant liquid refrigerant liquid thermal conductivity, density, viscosity and surface tension and related physical properties, and has a direct effect on boiling heat transfer coefficient. Thermal conductivity larger refrigerant, the heat resistance at the direction of heat transfer was low, the boiling heat transfer coefficient is large. Evaporator under normal working conditions, the refrigerant in the evaporator and the temperature difference of heat transfer surface, generally only 2 ~ 5 ℃, the intensity of the convective heat transfer depends on the degree of refrigerant liquid convection movement in the process of evaporation. Bubbles in the liquid boiling process, the internal movement, causes the liquid disturbance, this increases the possibility of a liquid parts in contact with the heat transfer surface, the liquid from the heat transfer surface absorption easier, boiling process more quickly. Density and viscosity of liquid refrigerant, by this disturbance is stronger, the greater the convective heat transfer coefficient. Density and surface tension of liquid refrigerant, the greater the bubble diameter is larger in the process of vaporization, heat transfer surface bubbles from generation to leave, the longer it will be less bubbles per unit time, heat transfer coefficient is small. Generally speaking, the thermal conductivity of freon is smaller than ammonia, density, viscosity and surface tension are bigger than ammonia, so its boiling heat transfer coefficient is smaller than ammonia. Refrigerant liquid wetting ability if the influence of the refrigerant of the heat transfer surface wetting ability is strong, is generated in the process of boiling bubbles is fine roots, can very quickly out of the heat transfer surface heat transfer coefficient is bigger also. Not well, on the other hand, if the refrigerant liquid wetting heat transfer surface, the formation of air bubbles root is very big, reduce the number of the vaporization core, even the heat transfer surface film formation, lower the heat transfer coefficient significantly. The commonly used several kinds of refrigerants are wettability liquid, but ammonia wetting ability than freon much stronger. Refrigerant boiling temperature refrigerant liquid boiling process, the influence of evaporator on the heat transfer surface per unit time the more the number of bubbles generated, the boiling heat transfer coefficient is larger. The number of bubbles per unit time is generated, and the heat transfer surface bubbles generated to leave the length of time, the shorter time, the more the number per unit time to generate bubbles. In addition, the smaller the diameter of the left wall if bubbles, the bubbles from generation to leave the time will be shorter. The size of the bubbles left wall, its diameter is by the buoyancy of air bubbles and the liquid surface tension balance. Buoyancy force bubbles left wall, and the liquid surface tension to prevent bubbles away. Bubble buoyancy and liquid surface tension and density difference (under the saturation temperature was The density difference between the liquid and vapor) The influence of. The buoyancy of air bubbles and is directly proportional to the density difference. The surface tension of the liquid in the direct ratio with the density difference of four times. So, with the increase of density difference, increases the surface tension of the liquid velocity, increase speed is much bigger than bubble buoyancy, the bubble can only rely on the expansion of the volume to maintain balance, so the diameter of the bubbles left wall. The size of the density difference related to boiling temperature, the higher the boiling temperature, the smaller the density difference between the saturated temperature, evaporation process will be more quickly, heat transfer coefficient is bigger. Above shows in the same evaporator, using the same kind of refrigerant, the heat transfer coefficient increases with the rising of boiling temperature. Evaporator structure liquid boiling process, the influence of air bubbles can only be produced in the heat transfer surface, effective heat transfer surface of the evaporator is a part of the contact with the refrigerant liquid phase. So, the size of the boiling heat transfer coefficient related to the structure of the evaporator. Experimental results show that the boiling heat transfer coefficient on the finned tube light tube, and greater than that of single tube bundle. This is due to add rib, the saturation temperature and the heating load per unit area under the same conditions, bubble formation and growth condition, finned tube than light tube. Velocity of bubble increase with increasing vaporization core number and reduce, make it easy to bubble from the heat transfer surface. The experimental results showed that the boiling heat transfer coefficient of finned tube bundle big light tube bundles. Have information, under the same saturation temperature, Rl2 boiling heat transfer coefficient of the finned tube bundles 70% larger than the light tube bundles, and R22 90% larger. According to above analysis, the structure of evaporator should guarantee the refrigerant vapor can quickly out of the heat transfer surface. In order to effectively use the heat transfer surface, should be the liquid refrigerant throttling to produce steam, before entering the evaporator is separated from the liquid, and in the operation management, flow of liquid refrigerant evaporator should be reasonable. In addition, the refrigerant oil, has some influence on the boiling heat transfer coefficient, and the influence degree is related to petroleum concentration. Generally said, when the refrigerant oil concentration is not more than 6%, not considering the influence of oil content is higher, can lower the boiling heat transfer coefficient.
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