| Improvement in design and performance of air-refrigerant heat exchanger has the great significant on development of industry and effective use of energy,because it has been widely used in all sorts of processes and mechanical equipment that need heat exchange.Therefore,develop the new heat transfer enhancement technique,condensation with liquid-vapour separation,and apply it to design,performance analysis and application of air-refrigerant condenser have some practical significant and economic benefit.Present work made detailed and in-depth theoretical and experimental research on thermodynamic performance and structure optimization of the condenser with liquid-vapour separation(LSC).Theoretical analysis included developing and improving the performance prediction models for LSC,which would be used in design of tube-pass arrangement and comparative discussion of the LSC thermodynamic performances.For the experiment,we would implement the replacement tests of variation liquid-vapour separations and refrigerants for LSC on the air-cooled heat exchanger test rig which met the national standards.Firstly,based on the principle of LSC design,that was,vapor condenses in short tubes and condensate is separated from liquid-vapour mixture between each short tub and drained away from the heat exchangers,developed a modelling solution for the heat transfer and pressure drop calculation of LSC,that was,calculated performances in short tubes and then process the mediate results and the input parameters,combined the results of all the tube-passes finally.We used the software,Visual Basic 6.0 and FORTRAN 90,to code two LSC models,one was a zone-by-zone model that just based on the heat transfer inside the tube,and another one was distributed parameter model,which considered the heat transfer in the airside as well and had ability to simulate the local thermodynamic performances.For the later model was more complex,reality and detailed,so it implemented two-dimension computational domain and subdivided technique in the windward area of the air-cooled condenser and used segment by segment method to calculate the finned tube.Because of the low refrigerant mass flow rate,the distributed parameter model also developed a correlation selected strategy based on identifying the pattern of the two-phase refrigerant flow.Moreover,model considers the effect of the refrigerant flow mal-distribution on the LSC thermodynamic performance as well,and proposed different algorithms to compute the flow distribution in the tube-pass on account of the inlet tube position.Two models showed well predictive ability,during the calculation condition,compared with the experimental data,the root-mean-standard(r.m.s.)deviations of the predictive heat transfer rate and the predictive pressure drop of the distributed parameter model were just 7.5%and 20.6%,respectively.The predictive precision of distributed parameter model were improved by 1.3%and 5.5%compared to the zone model,that were 8.8%and 26.1%.Thus,the distributed parameter model considered more influencing factors and became more complex and multifunction,but it had none of distortions.During the processing of the model developed,the zone-by-zone model was used to optimize the tube-pass arrangement for a LSC just contained condensation heat transfer,the PF proposed based on the second law of thermodynamics as criterion,and the distributed parameter model was applied to evaluate the tube-pass arrangement of LSCs included superheat,condensation and sub-cooling areas using the overall performance of condenser.Analysed the results before and after optimising,during the calculation condition,the PF values and the overall performance were improved by 110.4%?236.5%and 5.0%?9.0%,repectively.In addition,the distributed parameter model was used to analysis the effect of the inlet tube position of the first tube-pass,the inlet refrigerant flow rate and the inlet quality on the thermodynamic performance of the current tube-pass.The results revealed that the inlet tube position had greater influence the heat transfer rate but the pressure drop.Before and after the optimization of the inlet tube position,the heat transfer rate just had about increased by 0.6%,but the pressure drop decreased by a massive 85%.From the comparative investigation on the performance of the multi-pass and parallel-flow condensers(MPFC)with and without liquid-vapour separation,the average heat transfer coefficient of MPFC had about 34.6%improvement if it equipped the liquid-vapour separation.Then the local thermodynamic parameters of the two condensers were analysed comparatively,it was found that the mainly heat transfer enhancement came from the downstream of the MPFC without liquid-vapour separation,whose here has lower quality and heat transfer ability.The best enhancement tube-pass was improved by a massive 52.3%.Present work also did some effort to raise the computation accuracy,stability and efficiency of the distributed parameter model.The distributed parameter model using the Effectiveness Number of Transfer Unit(?-NTU)method was proposed to calculate the heat transfer.For the calculation errors and unable convergence of the program caused by the transition segment,a new method named segment self-subdivision was introduced to trace the phase change boundary.Besides Genetic Algorithms(GAs)was adopted as the calculation algorithm to determine the refrigerant side flow rate distribution as well to improve the old version.The new algorithms represented more universal,less complexity,stronger stability and quicker convergence.Based on the theory analysis above-mentioned and the optimization results,a trail-manufacture LSC was used to experimentally research on the influence of variation liquid-vapour separation and refrigerant type on the thermodynamic performance.The experimental results showed that the condensate leak area ratio of the second baffle with liquid-vapor separation had significant effect on the whole heat transfer and pressure drop of the whole condenser.In addition,In the replacement of the refrigerant R22 with R410A,when the operating condition was not changed,because that the liquid-vapour separation applied to R22 was not suitable to R410A and the effect of the liquid-vapour separating deteriorated and failure,the pressure drop of the LSC increased by 32%?47%,the average heat transfer coefficient deceased by 11%?17%.Overall,when the LSC need replacement another kind of refrigerant,the liquid-vapour separation should be re-design and replaced as well. |
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