| Air source heat pump is a kind of building cooling and heating equipment with mature technology and wide application.It has many advantages such as energy saving,high efficiency,safety and environmental protection.However,when the air source heat pump unit is heating in the low temperature area in winter,the outdoor finned coil evaporator is prone to frost,which leads to the problems such as the reduction of system heat production and the instability of heating supply.Therefore,it is of great significance to study the frosting and heat transfer characteristics of the outdoor finned coil evaporator under the condition of low ambient temperature for the optimization and improvement of the heating performance of the heat pump in winter.In this dissertation,the method of combining simulation and experiment is studied.Firstly,the frost formation and heat transfer characteristics of multi-row heat exchangers with different fin pitch structures are simulated by heat exchanger simulation software under low temperature conditions,and the performance differences of variable-pitch heat exchangers with different fin pitch combinations are mastered.Then customize the finned coil heat exchangers with different fin pitch,the frost experimental facility of variable fin pitch heat exchangers was designed and build.A comparative study was carried out on the multi-row finned coil heat exchanger with different number of rows and different fin pitch under the same low ambient temperature condition analyzing their amount of frost in low temperature condition,analysing the differences between the frost thickness and heat transfer performance;Finally,according to the combination of fin pitch with better comprehensive performance obtained from the experiment,the variable-pitch finned coil evaporator was made and applied to the air source heat pump test prototype for comparison test.Meanwhile,the flow field is optimized by adjusting the fan steering,the top-suction type test prototype was made for comparison test with the conventional prototype.The main research conclusions are as follows:(1)Through the 7 groups of different fin pitch of finned coil heat exchanger simulation research,the results show that on the mass of frost,frost mass of heat exchanger increases over time,and the frosting rate will decrease gradually,when t=60min,the frosting mass of sample 2 is decreased by 5.7% with the increase of 1mm fin pitch compared with that of sample 1,and the frosting mass of sample 3 heat exchanger with the increase of 2mm fin pitch is decreased by 16.7% compared with that of sample 1;when the fin pitch increases to a critical value(approximately 4mm)can be greatly reduced the subtotal frost mass,but continue to increase the fin pitch for the extent of decrease in the amount of frost is not obvious;The heat transfer capacity of the heat exchanger with smaller fin pitch is larger at the initial frosting stage,but the heat transfer capacity of the heat exchanger with smaller fin pitch will decrease significantly with the frosting process,while the heat transfer capacity of the heat exchanger with larger fin pitch will decrease slowly,and the heat transfer capacity of the heat exchanger with larger fin pitch is greater than that of the heat exchanger with smaller fin pitch in the middle and late frosting stage.The heat transfer coefficient of the heat exchanger with small fin pitch is lower and decreases faster,and the heat transfer coefficient of the heat exchanger with large fin pitch is higher.(2)Two rows and three rows variable fin pitch finned coil heat exchanger of the experiment results show that increasing the fin pitch of windward side finned tube row have inhibition to frost mass and frosting speed,in the same time,the frost mass of the two rows of heat exchangers on the windward side with fin pitch of 3mm,4mm and 5mm was reduced by 0.88%,8.57% and 10.77%,respectively,compared with that on the windward side with fin spacing of 2mm.The experimental data of three rows of heat exchangers also show that increasing the fin pitch on the windward side has the most obvious inhibitory effect on the frost mass and frosting speed,while changing the fin pitch on the third row has the least effect on the frost mass.Raising the fin pitch of the tube row of heat exchanger is beneficial to prolong the frosting and blocking time of the tube row on the windward side,and the effect of increasing the fin pitch of the tube row on the windward side is the best,followed by the second row and the third row.The frost crystals in the early frosting stage can improve the heat transfer capacity.After a period of time,the heat transfer capacity of the heat exchanger begins to decline,and the heat transfer capacity tends to be stable in the late frosting stage.The heat transfer capacity of the heat exchanger with small fin pitch is the highest at the initial frosting stage,but it decays rapidly afterwards.The variable fin pitch heat exchanger is less affected by frosting and the heat transfer capacity fluctuates less.The heat transfer performance of the variable fin pitch heat exchanger is better than that of the fixed fin pitch heat exchanger in the middle and late frosting stage.The heat transfer coefficient of the fin coils heat exchanger with fixed fin pitch fluctuates greatly,and the heat transfer coefficient is lower than that of the fin coils heat exchanger with variable fin pitch.The larger the fin pitch is,the smaller the heat transfer coefficient changes will be.(3)The prototype experimental of three-row fin coil evaporator with variable pitch and the conventional prototype with fixed pitch were compared under low temperature condition,the result shows that,compared with the fixed fin pitch unit,the heat production of the variable fin pitch prototype is increased by about 6.01%,COP is increased by 6.42%,while the defrost cycle extended 19 min,variable pitch finned coil evaporator has better heating performance and energy efficiency under the low ambient temperature.The comparison test between the top suction prototype and the conventional unit shows that the top suction prototype with optimized flow field increases the heat production by 7.76%,COP by about5.88%,and is beneficial to reduce the compressor shell temperature. |
PDF Full Text Request