بررسي تجربي انتقال حرارت چگالشي جريان R-134a داخل لوله موجدار پيچشي با شيب هاي مختلف

چكيده انگليسي :

Experimental study of Condensation heat transfer of R 134a flow inside a corrugated tube with different tube inclinations Davood Khoeini D khoeeni@me iut ac ir Sep 12th 2011 Department of Mechanical Engineering Isfahan University of Technology Isfahan 84156 8311 IranDegree M Sc Language FarsiAhmad Saboonchi ahmadsab@cc ut ac irMohammad Ali Akhavan Behabadi akhavan@ut ac irABSTRACT This experimental study addressed condensation heat transfer coefficient of R 134a flow insidecorrugated tube of different inclinations relative to horizon The experimental apparatus was basically avapor compression refrigeration cycle equipped with all necessary measuring instruments The testcondenser was a cross flow heat exchanger in which R 134a vapor flowing inside the inner corrugatedcopper tube was condensed by rejecting heat to the cooling water flowing in annulus The length of this testsection was 700 mm The empirical data was taken in the process of R 134a vapor condensation in sevendifferent inclinations of test condenser within the angular limit of 908 to 90 and four different massvelocities of 87 142 197 and 253 kg s m2 The analysis of collected data showed that the change in tubeinclination has a significant effect on condensation heat transfer coefficients This effect is morepronounced at low mass velocities and low vapor qualities At low mass velocities and low vapor qualities the highest coefficient of condensation heat transfer is achieved for the tube with inclination angle of 308 which is 41 more than the least heat transfer coefficient gained for the tube with 908 while theaverage heat transfer coefficientin test condenser with 30 is 31 4 more than that for 90 Also at low vapor qualities condensation heat transfer coefficient for 30 is 11 4 more than that for horizontal tube while the average heat transfer coefficient h in test condenser with 30 is 6 more than that forhorizontal tube In fact interfacial shear stress is dominant at high vapor velocity and high vapor quality while at low vapor qualities the vapor phase motion is slowed down and this would result in decrease ofinterfacial shear stress and consequently lower inertia forces near the wall Therefore the effect of gravityforce on the condensation of R 134a flow at low vapor qualities is more considerable For all massvelocities the highest average heat transfer coefficient is belonged to the tube with 30 The other point is that the tube with an inclination of 90 has the least heat transfer performance In downward verticalflow since the interfacial shear stress and the gravity are acting in the same direction the flow pattern willremain in annular status Besides the condensed liquid on the wall is thickened moving towards the flowdirection which leads to an increase in thermal resistance and consequently a decrease in heat transfer Likewise the interfacial turbulence will be minimized which is also another reason for the poor heattransfer performance For the vertical upward flow 90 due to interfacial turbulence at low vaporqualities there is better heat transfer performance in comparison to downward vertical flow This result isreversed at high vapor qualities and heat transfer performance for the downward vertical flow is better thanthe upward one This could be due to the better condensation on corrugated surface of the tube incomparison with the vertical upward flow Finally based on the present experimental results the newempirical correlation is developed to predict the condensation heat transfer coefficients in corrugated tubes This correlation predicts the experimental data within an error band of 9 Key words Heat transfer increase Condensation flow Corrugated tube Tube inclination

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بررسي تجربي انتقال حرارت چگالشي جريان R-134a داخل لوله موجدار پيچشي با شيب هاي مختلف

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