| The air source heat pump system(ASHPS)is prone to encounter frosting when operating at low temperature and high humidity working conditions in winter.The frost layer on the outside heat exchanger increases the heat exchange resistance between air and refrigerant,and results the decline of heat exchanger and system performance.On the other hand,the frost layer blocks the fin passage,which causes the decrease of fan flow rate and system stability,and may even cause safety problems of system operation.Therefore,it has become an important research topic to explore effective measures to inhibit frosting and efficient and feasible defrosting methods in the field of ASHPS.With considering the frosting process and low adhesion characteristic of superhydrophobic surfaces to condensed droplets,a frost suppression method based on synergy of superhydrophobic modification and airflow is proposed.In this method,the unfrozen condensation droplets on superhydrophobic surface were removed by high-speed airflow,it destroy the growth matrix of frost layer and strengthen the frosting inhibiting properties of the superhydrophobic surface.In this paper,the growth and removal behavior characteristics of condensation droplets on superhydrophobic surface were obtained by theoretical analysis,numerical simulation and experimental method.The frost suppression mechanism and characteristics of the novel frost suppression method were studied,and the coupling mechanism between the synergistic effect and the heat pump system was revealed.It provides theoretical support and method reference for promoting the frost suppression technology on surface modification.The main research contents and results are as follows:In terms of droplet growth,the growth dynamics and characteristic parameter evolution of condensation droplet clusters at the initial stage of frosting on superhydrophobic surface were studied.The equivalent thermal resistance model for a single droplet was established,the competitive adsorption effect of water vapor between droplet under frosting condition was proposed,and the corresponding equivalent thermal resistance correction coefficient was obtained by visualization experiment.It revealed the heat and mass transfer characteristics of an individual droplet during growth.With the analysis of system free energy,the deformation and jumping models for droplet coalescence were established,the unsteady simulation algorithm of droplet cluster growth was developed,and the dynamic simulation of droplet cluster growth on the superhydrophobic surface at the initial stage of frosting was carried out.On this basis,the influencing mechanism of the air state parameters,surface wettability and coalescence behavior on the growth characteristics of clusters was revealed.The results show that,compared with the contact Angle and substrate temperature,the growth rate of a single droplet is more sensitive to air humidity.For the growth of droplet clusters,the surface wetbility is an important factor affecting the growth characteristics,and it affects the characteristic parameters of the clusters by affecting the initial radius of jumping.In terms of droplet removal,the movement behavior and unsteady heat and mass transfer mechanism of evaporation for droplet under the action of air flow on superhydrophobic surface were studied.The basic modes of droplet movement on superhydrophobic surface were proposed,the changes of system free energy in movement process and the force taken of droplet under the action of air flow were analyzed,and the effects of surface wetbality and droplet size on the movement mode and corresponding critical speed were revealed.For droplet evaporation,the VOF multiphase flow model of droplet evaporation on superhydrophobic surface was established,the quasi-steady state CFD simulation method of multi-stage modeling was proposed,and the simulation results were verified by visualization experiments.On this basis,the evaporation characteristics of droplets in the velocity boundary layer of the air flow on the superhydrophobic surface were studied,the influencing mechanism of the the thickness of the boundary layer,the surface characteristics,the substrate temperature and the air temperature and humidity on the evaporation rate was revealed,and the effect of the evaporation mode on the survival time of droplets was obtained.The results show that the air velocity required to remove the droplets on the superhydrophobic surface with a contact Angle of 150° is about9.4m/s,the evaporation process of the residual droplets within airflow will complete within 20 seconds,which provides possibility for quickly remove of the condensation droplets on the superhydrophobic surface by airflow.In terms of frost suppression characteristics of the synergistic method,the synergistic effect of superhydrophobic modification and cold airflow on the performance of heat exchanger were studied.The airflow parameter calculation method based on droplet growth and movement model was developed,and the effects of environmental parameters,substrate temperature and fin-surface wetbality on airflow parameters and energy consumption were revealed.An experimental platform of finn-tube heat exchanger was constructed,the influence of the synergistic method on the heat exchanger’s comprehensive performance was experimentally studied under the calculated flow parameters,and the frost suppression characteristics of the synergistic method were revealed.It verified the feasibility of the synergistic method on heat exchanger scale.On this basis,the simulation algorithm for the frosting and defrosting of the fin-tube heat exchanger was developed,and the influence of ambient temperature and humidity on the performance of the synergistic frost suppression method was studied.The results show that the frost suppression effect of superhydrophobic modification method is further enhanced by the airflow with little total energy consumption change.Compared with the case without anti-frosting mothod,the synergistic method can increase the total heat transfer by 43.4% and delay the blocking time by 100%,respectively,and reduce the heat consumption of defrosting by 33% at the same time.In terms of the coupling mechanism between the synergistic method and the ASHPS,the frost formation types of surfaces with different wettability were analyzed by phase change dynamics mothod,which validated the feasibility of the application of the synergistic method in ASHPS.On this basis,an ASHPS which can realize the synergistic method by refrigerant undercooling and airflow heating was proposed.The frost-system coupling model was established to simulate the frost-defrosting process of system under typical frosting conditions,and the operating characteristics and comprehensive performance of the system in a single frost-defrosting period were obtained.Then the influence of three operating modes on system performance is studied.The results show that compared with the mode without anti-frosting measures,the mode of superhydrophobic modification,superhydrophobic modification combined with cold airflow,superhydrophobic modification combined with hot airflow increase the comprehensive performance coefficient of the system by 8.6%,9.7% and 14.3%,respectively.Compared with the passive anti-frosting method which only adopts hydrophobic modification,the application of active measures(air flow and heating)further improves the comprehensive performance of the system under defrosting conditions.Finally,the effects of climate condition on the performance of the novel ASHPS were quantitatively studied with the frost-system coupling model.The main cities of China with frost risk in winter are divided by the system frosting rate,and the representative cities are selected to analyze its heating season system performance.The results show that the maximum improvement of the three operation modes on the system performance under design parameters can reach 4.0%,11.0% and 26.8% in the severe cold,cold,and hot summer and cold winter areas,respectively.The simulation and performance calculation results of the system in heating season of representative cites show that the operation mode of superhydrophobic modification combined with hot airflow can increase the heat capacity of the system by 8.5%～25%,and increase the comprehensive performance coefficient by 6.0%～17.2% in the main climate area with frosting conditions in China. |
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