•Literatures on frosting characteristics, correlations, and mathematical models are reviewed.•Mechanisms of the frost formation process and its influence are introduced.•Frost layer thickness ...measurement methods and all frost characteristics are summarized.•The existing gaps in the research works on frost are identified, with recommendations offered.
As a common physical phenomenon, frost deposition is inevitable and always has significant negative effects on several industry fields, such as aerospace, aviation, and heating, ventilation, air conditioning, and refrigeration. To accurately predict and control a frosting–defrosting cycle, there is a need to understand the interrelated heat, mass, and momentum transport phenomena within the frost and at the air–frost interface, which is a moving boundary condition. Consequently, during the past several decades, there has been a continuous effort to advance the understanding and modeling of frost formation on cold surfaces on the basis of experimental, semi-empirical, theoretical, and numerical approaches. To provide an overview of the analytical tools for scholars, researchers, product developers, and policy designers, a review and a comparative analysis of the available literature on frosting characteristics, correlations, and mathematical models are presented in this study. The mechanisms of the frost formation process and its influence will be first introduced, followed by the presentation of methods for the measurement of the frost layer thickness and the frosting rate. Then, the frost characteristics, including the accumulation, the density, the thermal conductivity and morphology, and the heat and mass transfer coefficients, will be summarized. The existing gaps in the research works on frost will be identified, and recommendations will be offered as per the viewpoint of the present authors. Finally, the conclusions of this study will be given.
•PCMs integrated building envelope and equipment in 2004∼2017 are reviewed.•Melting temperature range of PCMs used for envelope is 10∼39°C.•Melting temperature range of PCMs used for equipment is ...−15.4∼77°C.•PCMs’ positive effects on energy saving and thermal comfort are demonstrated.•The existing gaps in the research works are identified and classified as 5 aspects.
Confronted with the crises of the growing resource shortages and continued deterioration of the environment, building energy performance improvement using phase change materials has received much attention in recent years. This review work provides an update on recent developments, 2004∼2017, in phase change materials used to optimize building envelope and equipment. Firstly, a review of building envelope optimization methods by integrating surrounding wall, roof, and floor with phase change materials, is given. This is followed by reporting articles on building equipment optimized with phase change materials to reduce regular energy consumption. Series of air cooling, heating, and ventilation systems coupled with thermal energy storage were comparatively investigated. Finally, the existing gaps in the research works on energy performance improvement with phase change materials were identified, and recommendations offered as authors’ viewpoints in 5 aspects. It was also found that the phase change temperature range of PCMs used was changed from 10∼39°C for envelope to −15.4∼77°C for equipment. We believe this comprehensive review might provide an overview of the analytical tools for scholars, engineers, developers, and policy designers, and shed new light on the designing and performance optimization for PCMs used in building envelope and equipment.
•Frost retarding and defrosting studies published in 2000–2017 are reviewed.•Two types of 12 frost retarding measures are classified and analyzed.•5 defrosting methods and 6 improvement methods are ...summarized.•Initiation and termination control strategies of defrosting operation are presented.•The existing gaps in the research works are identified and classified as 5 aspects.
Air source heat pump (ASHP) units have found worldwide applications due to their advantages of high energy efficient and environmental friendly. Frost deposition and accumulation on the surface of the outdoor coil in an ASHP unit is inevitable and always play significant negative effects. To accurately predict and control a frosting-defrosting cycle, the interrelated heat, mass, and momentum transport phenomena within frost, melted frost and at the air-frost interface, a moving boundary condition, should be clearly understood. This review paper focuses on the developments in frost retarding and defrosting investigations for ASHP units from 2000 to 2017. 12 frost retarding measures and 5 defrosting methods are firstly introduced, followed by 6 typical system optimization methods during reverse cycle defrosting. Alternative control strategies to start and end a defrosting operation are thereby presented. Basing on previous analysis, the existing gaps in the research works on frost retarding and defrosting are identified, and recommendations are finally offered as per the viewpoint of the present authors. This comprehensive and systematic review around an entire frosting-defrosting cycle might provide an overview of the analytical tools for scholars, researchers, product developers, and policy makers, and shed new light on the designing and performance optimization of ASHP units.
•A numerical study was conducted on a task/ambient air conditioning system.•The effects of supply vane angle on energy utilization was studied.•The effects of supply vane angle on thermal comfort was ...investigated.•TOPSIS method was effectively used for combined evaluation in buildings.•The best supply vane angle of 30° (EUC=1.28, DR=8.89) was obtained.
In subtropical area, air conditioning (A/C) is widely used to provide a suitable thermal indoor environment. During operation, many parameters or configurations may influence the performance of the A/C system. Among them, the supply vane angle is an important factor, which can influence the energy saving and thermal comfort level in the occupied zone, according to previously related studies. However, it was revealed that it was difficult to get a balance between these two aspects. Therefore, based on the previous study, this further study was conducted aiming to determine the suitable angle to achieve the best performance. Hence, the technique for order preferences by similarity to an ideal solution (TOPSIS) method was used to calculate the combined performance considering energy saving and thermal comfort. Finally, the best supply vane angle of 30°, with the energy utilization coefficient (EUC) and draft risk (DR) at the values of 1.28 and 8.89, was obtained, at which the A/C system achieved a medium energy saving performance and a lower draft risk. It’s indicated that using TOPSIS method can help tackle with conflicting effects for A/C systems in buildings.
•Four cases were conducted with two- and three-working-circuit outdoor coils.•The melted frost effect was quantitatively evaluated with eight parameters.•Defrosting efficiency increased from 48.34% ...to 58.79% after outdoor coil enlarged.•The MES effect was changed from 0.77% to 1.49% after outdoor coil enlarged.•Future work around defrosting topic was discussed at different aspects.
When an air source heat pump works at reverse cycle defrosting mode, the metal temperature of its indoor coil decreases and that of outdoor coil increases. The energy storage of two coils’ metal at heating mode would influence the system defrosting efficiency. For a vertically installed outdoor coil having multiple circuits, meanwhile, the melted frost downwards flowing from upside circuit(s) also delay the defrosting process of downside circuit(s). To quantitatively evaluate the influence from two parameters, metal energy storage and melted frost, four experimental cases were designed and carried out in this study, based on a specially made three-circuit outdoor coil. Three circuits are working at the frosting mode, while two and three are used at the defrosting mode in four cases. As concluded, the melted frost influence on defrosting efficiency and metal energy storage effect would be increased from 4.87% to 10.45% and from 0.77% to 1.49%, respectively, after the outdoor coil changed from two- to three-working circuit. Outcomes of this study are meaningful for the structure optimization and energy saving of air source heat pump units at defrosting mode.
•Comprehensive numerical method on supercooled droplet impingement pertinent to aircraft icing.•Time-dependent solidification proportion and final frozen shape of droplets.•Concept of activity ...duration to investigate the effect of LWC.•Logarithmic correlation for predicting transient heat transfer through the wall.
In-flight icing usually occurs when supercooled droplets impact on the cold surface of an aircraft, which should be concerned for its adverse effects on aerodynamic performance. To fundamentally elucidate the detailed mechanism of aircraft icing, a mathematical model on the impingement dynamics and solidification of a supercooled water droplet was developed and further validated with previous experimental results. Considering the effects of surface tension, wall adhesion and contact line dynamics, the coupled volume of fluid and level-set method was used to track the air–water interface, with the solidification issue solved by the enthalpy-porosity method. The temporal evolutions of the water phase, flow velocity, temperature and heat flux distributions were tracked and analyzed after the phase transition of supercooled water occurred. High impact velocities and millimeter-sized droplets were considered in this study to make the results more applicable to in-flight icing. As concluded, the spreading ratios of the droplets mainly distribute in the range of 0.8 ± 0.1 corresponding to LWC = 1.0 g/m3. Besides, the transient heat transfer between the solid surface and droplets could be fitted by a logarithmic function after appropriate dimensionless processing, which was proportional to the 1.5th power of the liquid fraction. Contributions of this work could be an effort to understand the microphysical phenomenon in the aerodynamic icing process.
•Afield test was carried out for deep boreholes with 2000m depth.•The performance of deep borehole ground source heat pump was evaluated.•A 3D numerical model according to geological structure was ...developed.•The simulation results showed a good agreement with the field test data.•The impacts of inlet velocity, inlet temperature and flow pattern were examined.
Deep borehole ground source heat pump (DBGSHP) is a new type of heat pump heating system which extracts deep geothermal energy through heat exchange and can be applied for space heating in winter. To date, the development of deep borehole heat exchangers (BHEs) is limited to the cognized structure design and there is a lack of the experimental studies. This paper presents the investigation of the heat transfer characteristics of the heat exchanger of a DBGSHP heating system through both field test and numerical simulation. A field test was first carried out based on the DBGSHP implemented in a demonstration project. A numerical model was then developed to facilitate the evaluation of the heat extraction capacity and the outlet temperature of the coaxial deep BHEs. Based on the numerical model developed, a sensitivity study was further performed to examine the effect of the primary parameters including the inlet velocity, inlet temperature, flow pattern (one was that the circulating fluid flowed from the inner pipe to the annular space and the other was that the circulating fluid flowed from the annular space to the inner pipe) and pipe diameter on the performance of deep BHE. The results from the field test indicated that the average heat transfer capacity of each single borehole, the average COP of the heat pump unit and the DBGSHP heating system COP were 286.4kW, 6.4 and 4.6, respectively. The simulation results matched well with the field test data, and showed that the inlet fluid velocity between 0.3m/s and 0.7m/s as well as the circulating fluid flowed from the annular space to the inner pipe can result in a better performance for the system of concern. The results from this study could be used as a reference basis for optimal design of coaxial deep BHE and to promote the utilization of deep geothermal energy.
•Negative effects due to gravity eliminated when outdoor coil horizontally installed.•Defrosting efficiency increased 9.8% after outdoor coil horizontally installed.•Defrosting efficiency decreased ...6.6% when air fan reversed to blow the melted frost.•Total mass of the retained water collected decreased 222g less after wind blowing.•Higher DEV respected better defrosting performance for multi-circuit outdoor coils.
When frost forms and accumulates over the outdoor coil’s surface in an air source heat pump (ASHP) unit, system operating performance will be dramatically deteriorated. Reverse cycle defrosting is the most widely used standard defrosting method. A previous related study reported that downwards flowing of melted frost due to gravity over a vertical multi-circuit outdoor coil would decrease the reverse cycle defrosting performance. If the outdoor coil can be changed to horizontally installed, the flow path of melted frost over coil surface can be shortened, and the flow directions of refrigerant and melted frost changed from opposite to orthogonal. Consequently, a better defrosting performance is expected. In this paper, therefore, an experimental study on defrosting performance for an ASHP unit with a horizontally installed multi-circuit outdoor coil was conducted. Experimental results show that, when a vertical outdoor coil was changed to horizontally installed, the defrosting efficiency increased 9.8%, however, with the same defrosting duration at 186s. Furthermore, when the outdoor air fan was reversed to blowing the melted frost during defrosting, the total mass of the retained water collected decreased 222g. However, the defrosting efficiency was not increased, but decreased 6.6% because of the heat transfer enhanced between hot coil and cold ambient air.
•Energy transfer procession in an ASHP unit during defrosting was explored.•Effect of metal energy storage on defrosting was proposed and calculated.•Metal energy storage effect was changed from ...positive (0.33%) to negative (−2.18%).•Defrosting efficiency was improved about 6.08%, from 42.26% to 48.34%.•Contributions of this study can guide the design optimization of two ASHP’s coils.
Air source heat pump units have found their wide applications in recent decades due to their high efficiency and low environmental pollution. To solve their undesired frosting problem, reverse cycle defrosting is always employed. As a transient and nonlinear heat and mass transfer procession, defrosting performance directly affects the occupants’ thermal comfort. During defrosting, the metal energy storage values of indoor and outdoor coils are varied as their temperature fluctuations. It is therefore necessary to investigate the energy transfer procession in an air source heat pump unit and the effect of metal energy storage during defrosting. However, scarce of attentions were paid to this fundamental problem. In this study, two experimental cases with two-working-circuit and three-working-circuit outdoor coils were conducted basing on frost evenly accumulated on their surfaces. After four types of energy supply and five types of energy consumption during defrosting were calculated, a qualitative and quantitative evaluation on the metal energy storage effect was then given. As concluded, after the outdoor coil enlarged 50%, the metal energy storage effect can be changed from positive (0.33%) to negative (−2.18%). The percentages of energy consumed on melting frost and vaporizing retained water were both increased. Defrosting efficiency was improved about 6.08%, from 42.26% to 48.34%. Contributions of this study can effectively guide the design optimization of indoor and outdoor coils and promote the energy saving for air source heat pump units.
•Ethanol-water mixture was used in a loop heat pipe for aircraft anti-icing.•The loop heat pipe with ethanol-water mixture obtained better performance.•Phase-change inhibition explains the better ...performance of the system with mixture.•60% concentration of mixture reduced response time of the system to 60.16 min.•60% concentration of mixture enabled the highest temperature distribution on wing.
Owing to the increasing demand of an energy-saving aircraft anti-icing technology and cooling of hydraulic system, loop heat pipes are gradually becoming efficient heat transfer mediums that can meet both anti-icing and heat dissipation requirements. To fundamentally investigate the heat transfer performance of a loop heat pipe used for aircraft anti-icing, a stainless steel-nickel one was fabricated and tested at three typical inclination angles under both steady and transient states, with the system performance as the final evaluation index. Additionally, to address the freezing problem under negative temperature flight conditions, ethanol-water mixture with four concentrations was specially used as the working fluid. The steady-state results showed that, 60% concentration of mixture enabled the loop heat pipe to obtain a lower operating temperature as 178.1 °C, and a smaller thermal resistance as 0.26 °C/W at 300 W. In transient tests, the loop heat pipe with 60% concentration of mixture operated robustly and stably, and reduced the total response time by 30.18%, about 26 mins, than in pure ethanol group. It also achieved the highest temperature distributions on the wing at −20°, which were about 31.8 °C higher than those in pure ethanol group. This study aims to effectively guide the utilization of ethanol-water mixture in loop heat pipes, and shows highly practical value in further applications of loop heat pipes in aircraft anti-icing.
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