| When the surface temperature of outdoor coil of an ASHP unit lower than 0? and the dewpoint temperature of ambient air,frosting occurs on the surface of its outdoor coil.Frosting deteriorates its operational performance and energy efficiency,and reduces its output heating capacity.To alleviate the negative impacts of frosting on ASHP units,studies on frosting mechanism and characteristics,methods for suppressing frosting,defrosting methods,and the operating characteristics for an ASHP unit during frosting and defrosting operations have been carried out.Although a large number of studies on frosting characteristics on FTHXs,the most commonly used structure for the outdoor coil in an ASHP unit,have been conducted and significant progresses made,the experimental investigations into the detailed characteristics of frost growth,especially frost distribution,on FTHXs are still considered insufficient,which is the main subject area addressed in the research project reported in this Thesis.To facilitate the research objectives of this project,firstly,an experimental setup,made up of four parts,used for investigating the characteristics of frost distribution and growth on FTHX was designed and constructed.The first was R410A refrigerant supply system with controllable flow and temperature(pressure)based on flow distribution characteristics of parallel pipelines,which can supply two-phase R410A with temperature range of-15?5? for frosting and superheated R410A with temperature above 50? for defrosting.The second was frosting environmental chamber,which can supply humid air with temperature range of-5?20? and relative humidity range of 40?95%.The third was wind box,which can effectively adjust the air flow passing through the FTHX uniformly.The last was visualization system,which can observe the local and global frost layer accumulated on the FTHX during the frosting period.Based on the technology of image binarization,the frost growth on the FTHX can be evaluated quantitatively.Secondly,the detailed frost distribution and growth characteristics for two experimental FTHXs with different fin pitches under a typical frosting condition were experimentally investigated.Based on the visualization results,frost on different areas of the finned tube heat exchangers were named and uneven distribution of frost along the air flow direction was studied in detail.The study results demonstrated that frost not only accumulated on the surfaces of fins and tubes of an FTHX,but also on its edge of windward fins.The frost on the surfaces of fins and tubes was not evenly distributed along the air flow direction(there is little frost on the surfaces of leeward tubes and fins along the height of tubes),and the frost on the edge of windward fins grew not only along the inverse direction of air flow but also along the length of tubes.Furthermore,the ratios of the frost mass accumulated on the edge of windward fins to that on the entire FTHX surface were13.7%(60 mins frosting duration)and 12.5%(120 mins frosting duration)for the two FTHXs,respectively.Thirdly,the effects of air temperature(Ta),relative humidity(RHa)and initial air face velocity(va,ini)on frost pattern,frost thickness,frost mass and averaged frost density on different parts of,and the overall frosting rate and air side performances of,two FTHXs with different fin pitches were experimentally investigated.The results showed that under the given environmental parameters,the protruded thickness of frost on the edge of windward fins and the ratio of the frost mass on the edge of windward fins to that on the entire FTHX were both increased with the increases in Ta and RHa,but decreased with an increase in va,ini.In addition,the effects of RHa on the mass ratio were more significant than those of Ta and va,ini.Furthermore,the averaged frost density on the edge of windward fins was increased with an increase in RHa,and slightly impacted by Ta and va,ini.The averaged frost density on the surfaces of fins and tubes was increased with an increase in va,ini but decreased with an increase in RHa,and slightly impacted by Ta.Fourthly,an experimental study on the differences in frost pattern,frost thickness,frost mass and averaged frost density on different parts of,and the overall frosting rate and air side performances of,an FTHX at fan control modes of constant air flow rate and constant outdoor air fan speed was carried out.The study results showed that compared to the use of the constant outdoor air fan speed control mode,the use of the constant air flow rate control mode can make the frost at the exit from leeward fins more compact and uniform,and lead to an accelerated frost growth on the surfaces of fins and tubes but an impeded frost growth on the edge of windward fins.At the end of a 60-min frosting duration,the protruded thickness of frost on the edge of windward fins,and frost thickness at the entrance to the windward fins at the constant air flow rate control mode were 7.9%and 11.1%,respectively,smaller than those at the constant outdoor air fan speed control mode.In contrast,the frost mass on the FTHX and the averaged frost density on the surfaces of fins and tubes at the constant air flow rate control mode were 6.7%and 16.7%,respectively,larger than those at the constant outdoor air fan speed control mode.Finally,based on the experimental results of the frost distribution characteristics of FTHXs,a novel mathematical model for predicting frost distribution and growth on an FTHX was proposed and experimentally validated.The model was made up of a frosting sub-model for the frost growth on the edge of windward fins and a frosting sub-model for the frost growth on the surfaces of fins and tubes.The former can predict the characteristics of frost growth on the edge of windward fins and the latter that on the surfaces of fins and tubes.In addition,the proposed frosting model can also precisely predict the air side performances of an FTHX under frosting conditions.The research project reported in this Thesis has made important contributions to better understanding the frosting mechanism and characteristics of FTHXs.The research results have made available both the essential data for the development of future numerical studies on frost growth,and the guidance to the optimized designs of defrosting strategies and frosting suppression measures for ASHP units,which can significantly enhance their operating performances and hence promote the application of ASHP units in frosting regions.The Thesis is a part of the Natural Science Foundation of China project"Research on the system innovation and operation law of solar assisted air source heat pump with convective phase change storage"(?51878209). |
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