Study On Performance Optimization Of Evaporator For R744/R290 Heat Pump System

It is urgent to take countermeasures for the regional environmental problems,especially the higher building energy consumption,induced by the intensified energy crisis.Building energy consumption accounts for a relatively high proportion of total social energy consumption,and thus heat pump system playing an important role in energy saving and emission reduction in the construction sector due to its particularity.The replacement of working fluid is the key point of energy saving in the field of heat pump.The advantages of mixed working fluid in environmental protection and thermal properties are further highlighted with the development of relevant technologies.Currently,researches on mixed-working heat pump systems have been focused on the substitution of working fluids,whereas few attentions have been posed to the development of key components.With the expansion of the application range of mixed working fluids in heat pump water heaters,it is significant to develop and improve the key components of mixed working fluids.In this paper,the performance of the evaporator for the environmentally friendly mixed working fluid R744/R290 heat pump system was optimized.The main works were conducted as follows:(1)The design process and scheme were formulated,according to the current problems of the evaporator.Correspondingly,the design calculation was carried out under the design conditions,and the optimal design was established through comparing the calculation results.(2)A steady-state distribution parameter model of the evaporator was established by comparing the experimental data and the simulated calculation data under the same working conditions.And then,both the accuracy of the model and the applicability of the heat exchange correlation formula to R744/R290 ware verified.(3)The structural size of the evaporator was further optimized using the established heat transfer analysis model.And the sensitivity analysis was performed to simulate the effects of different working conditions and parameters on the performance of the evaporator.Then the corresponding evaporator design and simulation calculation software were developed.Based on the design and simulation work of the evaporator for the R744/R290 heat pump system,main results are as follows:(1)The optimum program of the high-efficiency evaporator of the combined threaded pipe was established,through multiply comparing the three design schemes,and synthetically considering the heat transfer coefficient matching,consumables and pressure drop.(2)The comparison between experimental and simulated values showed that the established heat transfer analysis model was accurate and the selected heat transfer regression was applicable to R744/R290.(3)Under the simulated rated operating conditions,the performance of the evaporator is optimal when the outer tube of the two-phase zone is(37)22*1mm,the inner pipe is(37)12.7*0.75mm;the outer pipe diameter of the superheat zone is(37)25.4*1.3mm,and the inner pipe diameter is(37)12.7*0.75 mm,the length is 7.2m.Compared with the casing evaporator,the heat exchange capacity is increased by 7.13%,and the cold weight ratio is increased by 29.32%.(4)Both the heat transfer amount and heat transfer coefficient increased with either the increasing water,or the water inlet temperature flow in the selected working conditions.