Increasing Air Velocity Research Articles

Comparação de métodos para determinação do fator solar de vidros

Abstract Solar Heat Gain Coefficient (SHGC) is a thermal property of glass and transparent elements, defined as the ratio between the amount of solar energy that passes through the glass and the amount of solar energy that reaches it. SHGC can be determined using mathematical models, computer simulations, or field and laboratory measurements. This paper compares the methods for obtaining the SHGC, which include laboratory measurements using a portable calorimeter, computer simulation using the WINDOW software and calculated according to ISO 9050 standard. Two solar control glasses and one clear glass, used as a reference, were selected. The SHGCs obtained using laboratory measurements were lower than those obtained using the other methods due to the solar simulator and equipment limitations. Despite this, there was a good approximation of the behaviour of the glasses regarding the increase in air velocity. The heat transfer coefficient on the exterior surface was fixed, so the SHGC obtained for each glass based on ISO 9050 remains the same, regardless of the air velocity. The SHGC results based on ISO 9050 were similar to those measured by means of a portable calorimeter under conditions of higher air velocities.

Experimental and numerical research on a C-type heat exchanger in duct air conditioner under non-uniform airflow

The indoor unit of duct air conditioner typically employs an A-type heat exchanger (HX), but non-uniform airflow within the duct significantly degrades heat transfer performance. The current study proposes a C-type HX to accommodate the non-uniform airflow better. Experiments are conducted to compare the C-type and A-type HXs under varying conditions, including air volume flow rate, air velocity non-uniformity, inlet air temperature, and condensing temperature. Results indicate that as the non-uniformity of inlet air velocity increases, the heat transfer capacity of A-type HX decreases obviously. In contrast, the C-type is insensitive to the non-uniform air velocity distribution, which consistently outperforms the A-type in terms of heat transfer capacity throughout various operating conditions. Additionally, a numerical model is developed to compare the local parameters of the two HX types. The C-type demonstrates a more uniform outlet air temperature and exhibits a heat transfer capacity that is 19.1% higher than the A-type, while maintaining the identical heat transfer area and size parameters.

Implementation of thermoelectric wall systems for sustainable indoor environment regulation in buildings through numerical and experimental performance analysis

With buildings representing a substantial portion of global energy consumption, exploring alternatives to traditional fossil fuel-based heating and cooling systems is critical. Thermoelectricity offers a promising solution by converting temperature differentials into electrical voltage or vice versa, enabling efficient indoor thermal regulation. This paper presents a comprehensive investigation into the integration of thermoelectric wall systems for sustainable building climate control through numerical simulations and experimental analyses. Numerical simulations using computational fluid dynamics (CFD) techniques were conducted to model fluid flow and heat transfer within the thermoelectric wall systems under various operating conditions. These simulations provided insights into the system’s thermal behavior, which were validated through experimental setups designed to measure temperature differentials, airflow rates, and power consumption. The results showed that power consumption is directly correlated with electrical current, ranging from 0.19 W to 77.4 W as the current increased from 0.1 A to 2 A. Additionally, the heat absorbed by the system increased significantly with electrical current, by 706–1044%, depending on the air velocity. The thermal energy released from the hot side of the thermoelectric modules also rose substantially, ranging from 9850 to 5285% with increasing electrical current, and from 275 to 51% with higher air velocities. Moreover, increasing air velocity led to a 6.78–9.37% reduction in power consumption for currents between 0.1 A and 2 A. The coefficient of performance (COP) analysis revealed that optimizing both electrical current and air velocity is essential for maximizing system efficiency. While fan power consumption reduces COP at higher air velocities, neglecting fan power consumption results in COP improvements ranging from 6.5 × 10⁻⁴ to 49.0%. These findings highlight the potential of thermoelectric wall systems to enhance indoor comfort and energy efficiency in buildings.

Effect of air velocity variation on hardness vickers of 6061 aluminum TIG welding joints

Aluminum 6061, a commonly used metal, demands critical attention in welding due to its mechanical properties influencing structural strength. The welding of aluminum 6061 is affected by various factors, including air velocity conditions during the welding process. This research objectives to analyze Vickers hardness values in TIG-welded Aluminum 6061. The research focuses on TIG welding of aluminum 6061, analyzing the impact of air velocity variations in the welding environment on hardness values. The experimental design considers air velocity variations at 3.6 km/h, 4 km/h, and 5 km/h during TIG welding of aluminum 6061. Specimens from each research variable undergo Vickers hardness testing to analyze the correlation between air velocity variations and Vickers hardness values. Research findings reveal specimen 1 with an average hardness of 96 HV, specimen 2 at 105 HV, and specimen 3 at 110 HV. These differences depict hardness variations among specimens, emphasizing the complexity of air velocity variations' effect on welded joints' hardness. Hardness testing results consistently show the lowest hardness values at point number 2, while the highest values for specimens 1 and 2 are at point number 6. However, specimen 3 exhibits the highest hardness at point number 8. The research concludes that air velocity variations in the welding environment significantly impact hardness values in the welding results. Vickers hardness testing indicates an increase in hardness values with increasing air velocity, highlighting a proportional relationship between air velocity variations and hardness values

Experimental study and mathematical modeling of drying kinetics of mint leaves under forced convection

In this study, we investigated the influence of temperature and air velocity on the drying behavior of Mentha spicata L. leaves using a forced convection dryer. This drying method was chosen for its cost-effectiveness and its ability to facilitate fast and efficient drying while preserving the quality of aromatic herbs. Our aim was to identify the most suitable mathematical model for accurately representing the drying process. We analyzed experimental data on moisture content obtained during thin-layer drying of Mentha spicata L. using various established empirical models from existing literature. Thirteen empirical models were applied to fit the drying curves obtained. Statistical analysis, employing key parameters such as the coefficient of determination (R2), reduced chi-square (χ2), and root mean square error (RMSE), indicated that the Midilli and Kucuk model provided the most accurate fit for the observed drying curves. This was evidenced by the highest R2 values (0.9994, 1.0000), lowest χ2 values (6.07 × 10−6, 6.28 × 10−5), and lowest RMSE values (2.17 × 10−3, 6.86 × 10−3). We observed an increasing trend in moisture diffusivity with temperature variations between 32 °C and 60 °C, while maintaining a constant airflow velocity of 1.2 m/s. Additionally, an increase in mean air velocity from 0.7 to 1.7 m/s correlated with higher moisture diffusivity. The effectiveness of the Arrhenius equation in representing the temperature dependence of diffusivity was confirmed, with an activation energy of 72 kJ/mol, consistent with previous literature findings.

Sustainable Energy Production From Biomass: A Case Study On Tasmanian Blue Gum Eucalyptus Gasification

The global transition to renewable energy sources is a crucial response to the urgent need to reduce global warming and its negative environment consequences. One feasible approach within this paradigm is to generate steam from syngas produced by biomass gasification. Background: Gasification is a thermochemical process that transforms organic materials into combustible gas mixtures, provides a sustainable and renewable way to generate energy, chemicals and fuels. This technology helps to minimize greenhouse gas emissions and reliance on fossil fuels. Materials and methods: In this experiment, Tasmanian blue gum eucalyptus sawdust is used to generate pellets which will subsequently be processed into syngas using downdraft gasifier. The experiment investigates the effect of increasing air velocities specifically 10, 15, 20 and 25 M/sec as a gasifying agent to the gasification process. Results: The air velocity is measured at 10, 15, 20 and 25 m/s resulting in the time taken of 2280, 1860, 1560 and 1380 with efficiency increases of 41.11, 47.73, 49.62 and 54.67, respectively. This implies that optimizing air velocity is crucial for improving the overall performance of biomass gasification. The proximate and ultimate results of pellet are some better compared to the raw eucalyptus by researcher Filomena Pinto [16]. The efficiency is slightly high compared with the researcher R. Ravi Kumar [25]. The generated syngas may eventually replace the fossil fuel LPG for domestic purposes. Conclusion: The time taken for the gasification decreases with the increase of air velocity up to 25m/s. As the result the efficiency increases.

Numerical simulation and analysis on melting characteristics of a solid-liquid phase change process based on hot air riveting

To improve welding efficiency, a hot air riveting technology applicable to simultaneous welding at multiple stations is proposed, with the challenge lying in clarifying the melting state of the weldment. A numerical simulation method is proposed to investigate the melting state of the tag, using the Audi airbag tag as an example. The effects of various process parameters, including the exhaust duct distance, hot air temperature, hot air velocity, and the diameter ratio β, on the melting state of the tag are discussed. The melting mass, internal contour plots of the weldment and the melting rates are compared among the different parameter settings. The results indicate that under a heating temperature of 240 °C, the increase in hot air velocity has the most significant promotional effect on the melting of the weldment. Moreover, regardless of the velocity level, an increase in hot air temperature consistently enhances the melting of the weldment. Concurrently, an increase in the exhaust duct distance has an adverse impact on melting, yet this effect diminishes as the heating temperature rises. Finally, it is observed that at lower temperatures (200 ℃), diameter ratio β = 2.0 is more conducive to melting, whereas at higher heating temperatures (240, 280 ℃), β = 2.5 is more favorable for the melting of the weldment. This method effectively grasps the melting behavior of the weldment, enabling the selection of appropriate process parameters for hot air welding in actual production based on specific requirements.

Exergy and Energy Analysis of Forced Convective Thin-layer Solar Drying of Tiger Nuts (Cyperus esculantus L.)

Aims: To study the exergy and energy analysis for thin-layer solar drying of tiger nuts; and also to determine the drying kinetics for the thin-layer solar drying of tiger nuts. Study Design: Yellow variety of tiger nuts were dried in a forced convection solar dryer to determine the exergy and energy indices. Place and Duration of Study: Federal University of Technology, Owerri, Nigeria between October, 2022 to March, 2023. Methodology: The experiment was conducted at three different air velocities of 1.5, 2.5, and 3.5 m s-1 and two different loads of 70 and 140 g. The tiger nuts were dried from average initial moisture content of 52% (wb) to final moisture content of 10.2% (wb), at an average temperature range of 50.5 to 51.2 ºC. Energy utilization, exergy loss rate, exergy efficiency, improvement potential, and sustainability index were determined. Results: The results of energy utilization, exergy loss rate and improvement potential ranged from 2.1 to 5.09 W, 0.25 to 0.93 W and 0.08 to 0.54 W respectively; however, these values increased with increasing air velocities and loads. The results of exergy efficiency and sustainability index ranged from 41.8 to 68.49% and 1.72 to 3.17 respectively; these values increased with decreasing air velocities and loads. Graphical plots of moisture content with drying time showed that the moisture content of the tiger nuts decreased with increasing drying time. The characteristic nature of the curve is indicative of the fact that there was an initial higher rate of drying, which gradually declined with drying time. The results also showed that the drying rates of the tiger nuts ranged from 6.03 to 8.44 %MC/hr; and these values varied with the different air velocities and loads. Conclusion: The results from this research will serve as a guide in reduction of energy costs for the optimum design of convective dryers.

Heat transfer and power consumption of an optimized low-pressure compact drizzling cooling module integrated PEMFC stack radiator

The application of nozzle-assisted spray cooling to proton exchange membrane fuel cell (PEMFC) stack radiators has shown superior performance when compared with conventional air-cooled radiators. However, challenges remain with regard to high power consumption. This study experimentally optimized a low-pressure compact drizzling module coupled with a stack radiator as an alternative spray cooling strategy. The effects of the distance between the drizzling module and the radiator (30–90 mm) were examined, along with the influence of the spray flow rate/temperature, air velocity/temperature, and coolant flow rate on the thermal performance. The results showed that among the operating variables, the impact of the drizzling distance decreased with an increase in air velocity. Moreover, the drizzling performance was significantly influenced by the spray flow rate and air velocity, with minimal impact from the coolant-side variables. Optimal thermal performance was achieved at a distance of 50 mm and spray flow rate of 0.4 LPM, resulting in a 142 % improvement in heat rejection compared to an air-cooled radiator and a 132 % increase in the air-side pressure drop. Additionally, heat rejection only decreased by 7.3 % as the spray temperature increased from 40 °C to 70 °C. Two novel empirical correlations were developed, predicting heat transfer improvement and air-side heat transfer coefficient with over 85 % accuracy. These findings underscore the potential of drizzling cooling for stack radiators in fuel-cell vehicles, while the empirical correlations offer valuable insights for the development of spray-cooled radiators for next-generation electric vehicles.

Experimental study on the thermal performance of a high-temperature finned latent heat storage system

The primary objective of this study is to develop and evaluate the performance of a latent heat storage (LHS) system. A tube-in-tube heat exchanger configuration was used to design the LHS system. Sodium nitrate and air were employed as the phase change material (PCM) and heat transfer fluid (HTF), respectively. To mitigate the low thermal conductivity of the PCM, six circular fins were integrated on the HTF tube surface and experiments were conducted in an in-house experimental facility. Temperature plots from the charging-discharging experiments revealed that the HTF flow direction governs the heat transfer along the axis of the system. The sensible heat gain by the PCM in the range of 270–330 °C, led to an additional ⁓0.4 MJ of heat storage beyond the storage capacity of 1 MJ. Furthermore, increasing the inlet temperature from 400 °C to 440 °C reduced the charging time by 39.6 %, while reducing the inlet temperature from 210 °C to 170 °C decreased the discharging time by 18.2 %. Despite improved heat transfer, higher inlet temperatures adversely affected the thermal uniformity whereas, the increase in inlet air velocity enhanced both the dynamics of heat transfer and the thermal uniformity within the storage system.

Drop Retention and Departure in Adiabatic Shear Flow on Structured Superhydrophobic Surfaces.

Drops are retained or held on surfaces due to a retention force exerted on the drop by the surface. This retention force is a function of the surface tension of the liquid, drop geometry, and the contact angle between the drop and the surface. When external or body forces exceed the retention force, the drop begins to move. This work explores the conditions for which drop departure occurs on structured superhydrophobic surfaces in the presence of an applied shear flow. Drop departure is explored for five microstructured superhydrophobic surfaces, one nanostructured carbon nanotube surface and one smooth hydrophobic surface. Surface solid fractions range from 0.05 to 1.00, and measured static contact angles range from 121° to 161°. Droplet volumes of 5, 10, 20, 30, 40, and 50 μL are tested on each surface. For each experiment, increasing air velocity is applied to a droplet placed on a surface until the droplet departs. High-speed imaging is used to track droplet base length, height, cross-section area (as viewed from the side) and advancing/receding contact angles. Measurements of drop advancing and receding contact angles are reported at the point of departure, with increasing contact angle hysteresis observed prior to departure. Contact angle hysteresis is observed to be a good indicator of droplet mobility. Measurements of the average air velocity over the height of the droplet are determined at the point of departure for all conditions. The measured air velocity shows strong dependence on the surface solid fraction, and the required shear flow velocity decreases as the surface solid fraction decreases. This is most pronounced at very low solid fractions. A coefficient of drag for the departing drops in shear flow is calculated and is shown to decrease with increasing Reynolds number.

Cooling Air Velocity on Iron Ore Pellet Performance Based on Experiments and Simulations

During the pellet cooling process, cooling air velocity is crucial for optimizing the cooling rate, evaluating the utilization rate of cooling heat energy, and improving pellet performance. As the simulated cooling air velocity increased, the gas temperature at the cooling endpoint decreased from 87 °C to 51 °C, and the solid temperature decreased from 149 °C to 103 °C. The total enthalpy of the recovered gas initially reduced and then increased while the heat recovery rate gradually increased. During the experiment, the inhomogeneity of pellet quality gradually increased with the rise in cooling air velocity. The effect of cooling air velocity on pellet properties is primarily reflected in the formation of cracks and low-melting liquid phases (FeO and fayalite). As the cooling air velocity increases, the softening onset temperature of the pellet decreases significantly. The melting zone decreases from 193 °C to 105 °C, and the permeability of the adhesive zone increases.

Performance analysis of a novel air filtration and sterilization PV-Trombe wall

Bioaerosols have received widespread attention since the outbreak of Coronavirus Disease 2019 owing to their harmful effect on public health. To deal with indoor bioaerosols in an energy-saving method, an air filtration and sterilization PV-Trombe wall was proposed in this study. An experiment rig concerning thermal sterilization of aerosolized Klebsiella pneumoniae was set up to test its feasibility and effectiveness at different exposure temperature and residence time. With the experimental data, an inactivation model was derived based on the first kinetic model and Arrhenius equation. Besides, a mathematic model concerning heat and mass transfer was established for simulating the system performance at different conditions. The results reveal that: (1) The survival ratio of bioaerosols was 0.848, 0.689, 0.493 and 0.257 for the exposure temperature of 45 °C, 60 °C, 73 °C and 85 °C at the residence time of 6.5 s (2) the survival ratio predicted by the inactivation model corresponded well with the experimental results and the root mean square error was 0.041. (3) The electrical and thermal efficiencies were 0.134 and 0.218 while the indoor bacterial concentration was reduced by over 60 % with bacterial quantity of filter maintained at a low level. (4) The increase of air velocity could significantly improve the purification performance.

Permeability of granular mixtures under shear

Fluidised granular flows are gas–particle mixtures that occur in a wide range of industrial applications and geophysical flows. One factor governing the fluidisation behaviour of a granular flow is its permeability — the ability of the gas to move through the particle column. Although extensive research has been carried out on the gas–particle permeability under static conditions, most of the industrial and geophysical processes are dynamic phenomena, where parameters such as the shear rate change with both time and space. Here, the effects of shear rate on the permeability of gas–particle mixtures were studied through novel experiments where a granular column comprising particles of diameter 63–250 μm was sheared at shear rates between 0 and 213s−1 while an increasing flux of compressed air was introduced into the base of the column. Regardless of the shear rate, the granular column starts to expand upon increasing the air velocity, however, columns sheared at higher rates require greater air velocities to exhibit bubbling (and column expansion). We also instrumented the column with a pressure sensor. For the static column, as the air velocity increases, the pressure gradient across the granular column increases linearly until it reaches a maximum value, slightly decreases, and then plateaus. Conversely, for high shear rates, the pressure gradient continuously increases with increased air velocity, never reaching a maximum value or a plateau. The pressure gradient data were analysed alongside the videography in terms of the minimum fluidisation velocity and the minimum bubbling velocity, and both were found to be greater for higher shear rates. Consequently, our results show that increased shear increases the permeability of granular columns and the experimental data further suggests that in order to accurately describe the evolution of the pressure gradient across a sheared granular column, processes in the vertical as well as radial plane should be taken into account.

Study on the deposition characteristics of fine particles at local components in air conditioning ventilation ducts

Air conditioning ventilation ducts are widely used in contemporary buildings. The change of airflow direction in the ducts local components can easily lead to the deposition of particles, which can easily form a secondary source of indoor air pollution and have an adverse effect on human health. Based on this, this paper utilizes the existing experimental data to validate the numerical simulation. The Reynolds stress turbulence model combined with the method of enhanced wall function was used to investigate the deposition characteristics of fine particles in different local components. For the first time, local deposition rates and deposition hotspots were proposed to analyze the simulation results. The results show that the local deposition rate of the particles with a particle size of 1μm do not differ much on each wall surface, and the particles are deposited more uniformly on each wall at the local components, with a difference of less than 10 %, and the maximum local deposition rate is on the outer wall of the local components. When the particle size is larger than 10μm, the local deposition rate on the outer wall of the local component increases significantly, with a maximum increase of 23.3 % (compared to 5μm particles), and the maximum local deposition rate increases slowly with increasing air velocity, with a maximum increase of less than 8 %. The effect of increasing particle size on the maximum local deposition rate is much greater than the increase in air velocity. With the increase of air velocity and particle size, the deposition hotspots of both the 90° bend and the T-type tee duct migrate to the middle of the wall, and the deposition hotspots of the 4-way duct migrate to the inside of the wall.

Study on the influence of spray characteristics and air flow direction on heat transfer of spray evaporative condenser

Spray cooling is effectively utilized in evaporative condensers to achieve efficient heat and mass transfer transmission, while also effectively preventing performance degradation caused by packing blockage. The heat transfer efficiency is strongly affected by the relative flow direction between the spray and air flow. This work employs computational fluid dynamics to evaluate and examine the heat transfer effects of parallel and countercurrent air and spray flow in evaporative condensers. The findings indicate that increasing spray density and wall temperature enhances heat transfer efficiency. Smaller droplets with lower initial velocity exhibit superior heat transfer capabilities in parallel flow, while smaller droplets also perform well in countercurrent flow as long as the initial velocity is not too high. Additionally, a slight increase in air velocity improves heat transfer efficiency in both parallel and countercurrent flow conditions. It is important to note that the countercurrent condition has a larger heat transfer effect than the parallel flow.

Influence of Particle Surface Energy and Sphericity on Filtration Performance Based on FLUENT-EDEM Coupling Simulation

The adhesion of dust particles on the surface of the dust collector tends to cause great resistance to the dust collector and affects the operating efficiency. In order to visualize particles in the filtration process and to grasp the mechanism of particle viscosity and sphericity on filtration performance, a numerical simulation study was conducted to investigate the deposition behavior of particles during filtration, employing FLUENT-EDEM coupling technology. By examining the deposition process, the role of particle characteristics on dust behavior within the entire filtration system was elucidated. The effects of varying particle surface energy and particle sphericity on filtration pressure drop and cake porosity were analyzed. The findings reveal that under the force of the air, particles on the surface of the filter membrane experience compaction, leading to a reduction in the porosity of the formed cake layer. The diminution of porosity serves to impede the air, consequently augmenting the pressure drop across the filtration system and hindering the operational efficacy of the dust collector. As the surface energy of the particles increases, the adhesive forces between particles are intensified, leading to an elevation in the porosity of the cake layer and a subsequent decrease in the pressure drop. When the surface energy of the particles is increased from 0.01 J/m2 to 0.04 J/m2, the porosity experiences a modest increase of only 9.1%, yet the pressure drop is significantly reduced by half, amounting to a decrease of 1594 Pa. Under high particle surface energy, as filtration air velocity increases, particles are compressed, resulting in a decrease in cake porosity and an increase in pressure drop. Concurrently, our findings indicate that as the sphericity of particles increases, their surfaces become increasingly smooth which in turn results in a decreased porosity of the cake layer and, consequently, an elevation in the filtration pressure drop.

Analysis of melting behavior of layered different phase change materials for a cylinder insert into a channel

Phase change materials (PCMs) have diverse fields of application due to higher thermal inertia. The present study explores the melting behavior in dual-layered different PCMs in a cylindrical lithium-ion battery module placed in a long channel under the hot air stream. The main cylinder is packed with paraffin wax and covered PCM is chosen as calcium chloridehexahydrate (CaCl2-6H2O). The material between two PCMs is considered highly conductive and impermeable. The involved mathematical equations in two-dimensional forms are numerically simulated the applying the finite volume technique. The primary important parameters of melting behavior are the difference in temperature and inlet air velocity. The analysis is carried out for the effective parameters such as the inlet air temperature (Thot = 100 °C and 120 °C) and air velocity (սair = 2.5 and 5.0 m/s). The results revealed that an increase in air velocity reduces the highest and average temperature of the PCM. The changes in melting time of PCMs decline with time. Higher inlet air temperature also causes fast melting of the PCMs over time. The analysis revealed that the application of PCMs is undue when there is no phase change process. The analysis also revealed that an increment in the inlet air velocity causes a decrease in the average temperature. For the same air inlet temperature (Thot) higher air velocity leads to a slower rate of the melting process Therefore, melting time is less by ∼ 30 % (for the inner layer) and 32 % (for the outer layer) with decreasing air velocity. The findings of this study will help the designer to improve the battery thermal management system by modulating the temperature.

The dynamics of frost formation on road surfaces: an experimental analysis

This study delves into the complexities of frost formation on road surfaces, a phenomenon that presents significant safety hazards in transportation due to its sudden emergence and unpredictable nature. Despite advanced meteorological warning systems for snowfall and freezing rain, black ice and frost remain difficult to predict and counteract. To address this, a controlled indoor simulation experiment was designed to investigate the characteristics of road surface frosting at low temperatures.Results from the experiment indicated that growth of the frost layer thickness follows a parabolic trend with extended freezing time yet the mass of the frost layer increases at a roughly linear rate. The data also revealed that lower temperatures expedite the phase transition of water vapor to ice, leading to faster increases in frost layer height. Additionally, the effects of air temperature and velocity on frost properties were examined. Interestingly, higher air temperatures facilitated rapid frost formation initially, but later stages displayed a plateau phase in the rate of accumulation. Furthermore, increased air velocity up to 1 m/s resulted in greater frost mass, but higher velocities diminished frost formation due to enhanced heat transfer.In conclusion, this study offers a detailed analysis of frost layer development under controlled conditions, providing valuable insights into the environmental factors influencing this hazardous phenomenon. The findings contribute to the understanding of frost dynamics on road surfaces, with implications for improving predictive models and developing effective countermeasures for road safety during icy conditions.

Frosting characteristics of microchannel heat exchangers: Parametric studies and correlation development

Recently, microchannel heat exchangers (MCHX) have been widely used in heat pump systems due to their high surface-to-volume ratio and high efficiency of heat transfer. However, when the MCHX operates in frosting conditions, its performance is adversely affected. To study the frost formation and distribution characteristics of MCHX, a comprehensive parametric study was performed for the MCHX with a larger fin pitch. The effects of fin geometries and environmental parameters on the frosting characteristics of MCHX were studied. Fin height (Fh) can effectively inhibit frost formation, which is due to the increase of surface temperature with the increase of Fh. When Fh increased from 6.0 mm to 11.8 mm, the frost time increased by 30.5 %. However, the increase in Fh also brings the penalty of reduced heat transfer rate. The effect of fin depth (Fd) on frost formation performance is small because the frost is mainly deposited in the front part, resulting in the rear fin cannot work efficiently. Fin pitch (Fp) is the most significant structural parameter affecting frosting performance. When Fp increases from 1.6 mm to 4 mm, the frosting time increases by 4 times and frost mass increased by 2.2 times. This is because with the increase of Fp, the frost blocking in the front part can be effectively delayed, and the distribution uniformity of the frost layer is improved. Lower surface temperature and higher humidity ratio significantly promote frost formation and reduce the uniformity of frost distribution. When the inlet temperature is reduced from −5 °C to −8 °C, the frosting time is shortened by 2.15 times, and the frost mass is reduced by 55.7 %. When the relative humidity increases from 76 % to 92 %, the frosting time is shortened by 2.15 times, and the frost mass is reduced by 42.5 %. The air velocity has a minor impact on the growth rate of frost in the front part. However, as the air velocity increases, the uniformity of frost distribution is significantly improved. When the air velocity increased from 1.1 m s−1 to 2.2 m s−1, the mass of accumulated frost increased by 29.5 %. Furthermore, frost thickness and air-side heat transfer coefficient correlations were developed by testing 12 samples. The developed heat transfer coefficient correlation can accurately capture the changing trend of the j-factor. This study provides a fundamental understanding of the frosting characteristics of MCHX and can effectively guide the louvered fin optimization.