Wall Heat Transfer Coefficient Research Articles

Study on heat transfer behavior of CFS-PSB composite walls

The cold-formed thin-walled steel (CFS)-paper straw board (PSB) composite walls takes as the research object. The thermal performance of the wall was analyzed by the building heat transfer theory and the finite element analysis software ANSYS, explored the influence of relevant parameters on heat transfer coefficient of composite wall and proposed supplementary correction coefficient for applying the type of composite walls. The heat flux density field, temperature distribution field, node temperature, and heat transfer coefficient of the walls were also obtained. The specific parameters analyzed using this model mainly included the insulation material, web height, thickness of the section steel, width of the flange, length of the hole, number of hole rows, and horizontal and longitudinal spacing of the holes. The findings show that an increase in the web height reduces the heat transfer coefficient of the composite wall, which is conducive to improving the thermal insulation performance of the walls. The CFS-PSB composite wall with a web opening can significantly improve the heat transfer performance of the wall, reduce heat loss, and control the local thermal bridging phenomenon. The hole length, specific gravity of the openings, and number of openings were the main parameters affecting the thermal insulation performance of the walls.

Influence of the shock wave-turbulence interaction on the swirl distortion in hypersonic inlet

This study uses the Large Eddy Simulation (LES) technique to conduct an exhaustive analysis of the flow characteristics within the Rectangular-to-Elliptical shape Transition (REST) inlet under Mach 6 conditions. It mainly focuses on investigating the influence of the shock wave-turbulence interaction on the swirl distortion at the inlet exit. At the design condition, characterized by 0° Attack and 0° Sideslip, the incident shock wave at the inlet lip undergoes multiple reflections within the boundary layer of the domain wall, culminating in the formation of turbulent structures. The first reflected shock wave has the highest energy, exerting a significant impact on the boundary layer and the exit swirl distortion. On the contrary, the energy of the incident shock wave is progressively reduced due to repeated reflections, which results in reducing the exit swirl distortion. Under off-design conditions, characterized by 6° Attack and 0° Sideslip as well as 6° Attack with 6° Sideslip, variations in the incoming flow make the incident shock wave move inward, decreasing the frequency of shock wave reflections and even significantly reducing the reflected shock waves under conditions of 6° Attack and 6° Sideslip. However, this results in significantly increasing the exit swirl angle and distortion intensity. The obtained results demonstrate that changes in the incoming flow conditions significantly affect the level of exit swirl distortion by modulating the shock wave-turbulence interaction, especially in terms of the positioning of the incident shock wave and the quantity of reflected shock waves. In addition, this paper studies the wall heat transfer coefficient of the inlet. The obtained results show that the interaction between shock waves and the boundary layer significantly affects the heat transfer coefficient. This study provides a foundation for the comprehension and prediction of the performance of hypersonic inlets across a spectrum of flight conditions, and for the guidance of the design and optimization of such inlets.

Simulation of thermal radiation effects on the thermal properties of parietodynamic walls using an innovative model

The parietodynamic wall, a type of dynamic insulation, has been recognized as an effective technology to reduce energy loss in buildings by recovering heat energy through forced convection. However, current research on the thermal performance of parietodynamic walls has overlooked the influence of thermal radiation, a crucial factor in energy transfer within the air layers of these walls. To bridge this gap, an innovative simulation model was developed and experimentally validated. Employing simulation methods, we investigated the impact of thermal radiation on the thermal behavior of parietodynamic walls under various influencing factors. Our findings reveal that thermal radiation markedly increases heat loss. Specifically, at an emissivity of 1, thermal radiation contributes up to 80.7% to the heat transfer coefficient (HTC) of the parietodynamic wall. Moreover, for a parietodynamic wall without insulation, the HTC of this wall will increase by more than 268% when thermal radiation is taken into account, compared to when it is not considered. These revelations deepen our comprehension of the role of thermal radiation in parietodynamic walls and offer valuable guidance for the development of more energy-efficient buildings.

Flow boiling in a rectangular micro/mini channel with self-rewetting and non-rewetting fluids

In this experimental investigation the influence of self-rewetting during flow boiling was studied in a horizontal microchannel during one-sided uniform heating. The channel had a width and height of 6 mm and 0.3 mm respectively. Local wall temperature and direct in-time flow visualisation was performed. The working fluids under consideration included water as base reference, pure 1-butanol, pure ethanol, dilute butanol-water self-rewetting mixtures, and a dilute ethanol-water binary mixture. Tests were conducted at mass fluxes of 10, 15 and 25 kg/m2s, over a range of heat fluxes to produce both single phase and two-phase quasi steady state flow data. Within the wall temperature ranges of interest, the butanol-water mixtures (5 % and 7 % by volume butanol) had surface tension characteristics that would draw liquid toward local wall hotspots. Local and average wall temperatures and heat transfer coefficients were obtained and considered against the governing flow pattern visualisations. In contrast to the other fluids, the self-rewetting butanol-water mixtures exhibited significantly more stable flow and more predictable saturation flow boiling heat transfer coefficient trends across all heat fluxes under consideration. At the lowest mass flux, it was found that an increase in the heat flux resulted in an improvement in the self-rewetting fluids heat transfer performance, while at higher mass fluxes this dependence was diminished. Across all mass fluxes and heat fluxes the 5 % butanol-water outperformed its 7 % butanol-water counterpart. The tendency of self-rewetting to reduce local dry-out, even at low mass fluxes are demonstrated.

Regression modeling of Bödewadt slip flow dynamics involving Reiner-Rivlin nanofluid based on a modified Buongiorno approach

Regression models are useful in analyzing rotational flows as they enable accurate predictions of wall shear and heat transfer coefficient. In addition, Bödewadt flow is of paramount importance in fluid dynamics of rotating systems such as turbomachinery and geophysical flows. Moreover, nanofluid’s enhanced heat transfer properties can improve cooling efficiency in applications involving turbines and electronic systems. This study delves into the Bödewadt boundary layer flow of a Reiner-Rivlin fluid containing nanoparticles over a stationary porous disk under slip conditions. The two-phase Buongiorno model is employed, incorporating temperature-dependent diffusion coefficients for enhanced accuracy. To facilitate numerical simulations, the transport equations are converted into an ordinary differential system comprising four unknowns. In the present work, a highly reliable Keller-Box methodology is adopted which agrees very well with the MATLAB built-in program ‘bvp4c’. The computed 2-D and 3-D streamlines vividly capture the Bödewadt flow scenario with Reiner-Rivlin nanofluid. The principle aim to investigate the impact of non-Newtonian behaviour and slip on the flow pattern, while also examining the behavior of temperature/concentration field for nanoparticle working fluids. As thermophoretic diffusion increases, the thermal boundary layer thickens considerably, leading to a notable decrease in the cooling rate of the disk. In contrast, Brownian diffusion has only a minimal impact on the heat transport. In addition, wall suction effect is observed to significantly boost the disk’s cooling rate, though at the expanse of increasing skin friction coefficients. This study introduces linear and quadratic regression models designed to precisely predict both the surface drag and disk cooling rate, which are crucial factors in engineering processes.

Comparison of the effects on heat transfer through oil oscillation in a steel piston cooling gallery: Friction welding versus laser welding

Piston cooling gallery plays a critical role in managing the temperature of the piston and its performance. This paper aims to study the effect of oscillatory heat transfer within the cooling galleries, focusing on two distinct types: flanged (friction-welded) and smooth structures (laser-welded). To do so, the Eulerian multiphase flow model and the k-ω SST turbulence model were used to simulate the oil and air flow through the cooling gallery. Key parameters such as the oil filling rate, wall heat transfer coefficient, oil outlet flow rate, heat removal rate by the oil, and overall wall heat transfer rate were assessed for both gallery designs at an engine speed of 1800 r/min. The results indicate significant disparities between the two cooling gallery structures: the smooth structure exhibits an oil filling rate that is 5.41% higher and an oil outlet flow rate that is 33.48% greater compared to the flanged structure. The flange structure impedes internal oil movement, which leads to boundary layer separation, secondary wall contact, and the emergence of a double vortex phenomena. Collectively, these factors influence the oil fill ratio. This, in turn, affects the distribution of oil within the gallery and, subsequently, the efficiency of heat transfer. Comparative analyses of heat transfer within both galleries, under the same studied conditions, revealed that the smooth structure outperforms the flanged one. However, the presence of a flange significantly alters the heat contribution from different walls, thereby highlighting the substantial impact of flanges on the overall efficiency of heat transfer.

Determination of Local Heat Transfer Coefficients and Friction Factors at Variable Temperature and Velocity Boundary Conditions for Complex Flows

Transient conjugate heat transfer measurements under varying temperature and velocity inlet boundary conditions at incompressible flow conditions were performed for flat plate and ribbed channel geometries. Therefrom, local adiabatic wall temperatures and heat transfer coefficients were determined. The data were analyzed using typical heat transfer correlations, e.g., Nu=CRemPrn, determining the local distributions of C and m. It is shown that they are closely linked. A relationship lnC=A−mB is observed, with A and B as modeling parameters. They could be related to parameters in log-law or power-law representations for turbulent boundary layer flows. The parameter m is shown to have a close link to local pressure gradients and, therewith, near wall streamlines as well as friction factor distributions. A normalization of the C parameter allows one to derive a Reynolds analogy factor and, therefrom, local wall shear stresses.

Influence of flight acceleration on heat transfer and carbon deposits of aviation kerosene under supercritical conditions

During the acceleration or escape of the hypersonic vehicle, the flight acceleration will have an important impact on heat and mass transfer characteristics of pyrolytic hydrocarbon fuel in the regenerative cooling channel of scramjet. Based on the detailed pyrolytic mechanism and coking kinetics model, numerical model was established to study the flight acceleration effect on flow field, fuel pyrolysis, heat transfer and carbon deposit of supercritical hydrocarbon fuel in the horizontal cooling channel. Results indicate that the secondary flow is induced in the cross-section of horizontal cooling channels because of the flight acceleration and density gradient. Due to the secondary flow, thermal and fuel conversion stratifications in the radial distribution increase. The bottom wall heat transfer coefficient is higher, while the wall temperature and carbon deposits are lower than that of the top wall. Meanwhile, with the increase of the flight acceleration, the thermal and fuel conversion stratifications become larger. The differences of heat transfer coefficient, wall temperature and carbon deposits between the top and bottom walls rise. The operating pressure enhances the flight acceleration effect on the heat transfer and carbon deposits. In addition, Petukhov’s criterion is proved to be applicable to evaluate the flight acceleration effect quantitatively.

Effects of solar energy absorbed by south wall on carbon emission from space heating in hot summer-cold winter region

Carbon emission caused by space heating (CEH) in hot summer and cold winter climate zone has increased dramatically. Taking advantage of the solar energy absorbed by south wall (SEASW) can effectively reduce CEH, however, the accurate quantification of its impact remains a challenge. The study investigates the potential of SEASW in mitigating CEH. Key parameters affecting CEH, such as sunny day ratio, insulation form, and outer surface heat transfer coefficient of south wall (h), were analyzed. Results show the reduction in carbon emissions caused by the solar energy absorbed by each unit area of the south-facing wall (ΔCreduction) can be as high as 18.61 kgCO2⋅m−2 for the wall employing internal insulation and smaller h. A prediction model of ΔCreduction was further developed by using multivariate nonlinear regression, which can estimate the reduction in CEH for different fuels as heating source by a south wall in different sizes. Besides, the potential of SEASW on the reduction of carbon emission per unit floor area can reach 12.2 %. This study underscores the importance of SEASW as a carbon-neutral heating strategy, particularly for regions characterized by short winter heating periods and lower heating loads compared to colder climates. It provides valuable insights for sustainable energy practices and policy formulation.

Flow boiling characteristics of HFE-7000 in high aspect ratio microchannels with the effect of flow orientation

The aim of the present experimental study is to investigate the flow boiling characteristics in a high aspect ratio microchannel for two different channel orientations. Hydrofluoroether 7000 (HFE-7000) flowing within a rectangular glass microchannel (with a hydraulic diameter of 909 μm and a width-to-height aspect ratio of 10) coated with a tantalum layer to provide uniform heating along the channel was the configuration investigated. The data were recorded for mass flux of 14 − 42 kg m−2 s−1, while the heat flux was varied between 3.5 and 24.3kWm−2 under horizontal and vertical upward flow. Analysis of the experimental data reveals that thermal performance is influenced by both mass and heat fluxes, as well as flow orientation, and these are presented and discussed. The wall temperature mapping and heat transfer coefficient analysis showed that for low mass flow cases, the dry out occurs at a lower heat flux and for either flow orientation. At medium and higher heat fluxes, though, it is found that a horizontal flow produces dry out events more easily compared to vertical upward flow. The observed phenomenon during flow boiling also shows the importance of nucleate boiling and thin film evaporation on the heat transfer performance. In addition, pressure fluctuations during flow boiling were also recorded. A Fourier transform analysis of pressure fluctuations data showed that vertical upward flow produces a higher dominant frequency than horizontal flow. Finally, the analysis of thermal characteristics and pressure fluctuations can provide further insights into the design of devices where flow orientation plays a major role.

A two-phase flow node-solid joint simulation algorithm based on heat flow correction

In order to accelerate the temperature field calculation of fuel and tank wall protective materials, we propose a node-solid joint simulation method based on heat flow correction, in which the gas–liquid is used as a node for joint CFD (Computational Fluid Dynamics) calculation of the solid temperature field. Based on this method, we constructed a transient thermal analysis model of the fuel tank and performed a gas–liquid–solid conjugate heat transfer tight coupling simulation to obtain the wall heat transfer coefficient and the corrected heat flux under the specified thermal boundary conditions. The results show that the maximum relative errors of the average temperature of the gas–liquid node and the temperature field of the tank wall are less than 0.2 % and 6 %, respectively, and the calculation efficiency is about 160 times that of the traditional tight coupling calculation. When the solid boundary temperature is reduced by 200 K and increased by 500 K, the maximum relative error between the average temperature of the gas–liquid node and the solid temperature is less than 7 %. The algorithm has good robustness and accelerates the iterative design of the fuel tank. The accuracy of the algorithm was verified by the fuel temperature test experiment of the full flight profile.

Two-phase numerical simulation of thermal and solutal transport exploration of a non-Newtonian nanomaterial flow past a stretching surface with chemical reaction

Abstract Non-uniform heat sources and sinks are used to control the temperature of the reaction and ensure that it proceeds at the desired rate. It is worldwide in nature and may be found in all engineering applications such as nuclear reactors, electronic devices, chemical reactors, etc. In food processing, heat is used to cook such as microwave ovens, pasteurize infrared heaters, and sterilize food products. Non-uniform heat sources are mainly used in biomedical applications, such as hyperthermia cancer treatment, to target and kill cancer cells. Because of its ubiquitous nature, the idea is taken as our subject of study. Heat and species transfer analysis of a non-Newtonian fluid flow model under magnetic effects past an extensible moving sheet is modelled and examined. Homogeneous chemical reaction inside the fluid medium is also investigated. This natural phenomenon is framed as a set of Prandtl boundary layer equations under the assumed convective surface boundary constraint. Self-similarity transformation is employed to convert framed boundary layer equations to ordinary differential equations. The resultant system is solved using the efficient finite difference utilized Keller box method with the help of MATLAB programming. The influence of various fluid-affecting parameters on fluid momentum, energy, species diffusion and wall drag, heat, and mass transfer coefficients is studied. Accelerating the Weissenberg number decelerates the fluid velocity. The temperature of the fluid rises due to variations in the non-uniform heat source and sink parameters. Ohmic dissipation affects the temperature profile significantly. Species diffusion reduces when thermophoresis parameter and non-uniform heat source and sink parameters vary. The Eckert number enhances the heat and diffusion transfer rate. Increasing the chemical reaction parameter decreases the shear wall stress and energy transmission rate while improving the diffusion rate. The wall drag coefficient and Sherwood number decrease as the thermophoretic parameter increases whereas the Nusselt number increases. We hope that this work will act as a reference for future scholars who will have to deal with urgent problems related to industrial and technical enclosures.

Optimization of substation enclosure performance and carbon emission analysis based on OED

To explore the thermal performance parameters of substation enclosure parameters and interaction and influence degree, using the DeST software simulation and orthogonal test method to building load minimum as the target parameters, select external wall heat transfer coefficient, roof heat transfer coefficient, outer window heat transfer coefficient and orthogonal test factors, carry out the four factors three level L9 (34) Orthogonal test analysis, studied the influence of each factor and its interaction on the building load, and finally obtained the optimal combination of factors under the comprehensive boundary conditions. At the same time, the correlation between load and outdoor temperature in the optimal scheme of unmanned substation and the carbon emission results before and after optimization are also explored. The results show that through univariate analysis, with the heat transfer coefficient of exterior wall, roof and window, the cumulative cooling load is not linear. Through the orthogonal experimental analysis, the significance of substation building is> roof exterior wall> south Angle> window, and the optimal is A1-B1-C2-D2The annual cumulative building cooling load is 20165.59 kW h, and the peak cooling load appears on July 24. The operating condition of 0 ~ 9kW accounts for 90.6% of the annual operating time. There is a strong linear correlation between the building cooling load and the outdoor temperature (R2=0.65), the energy saving rate of the final optimized scheme is 17.71%, and the carbon saving rate is 26.72%. The research content of this paper provides reference for the renovation of existing buildings and the design of new buildings in substation buildings.

Demonstration of a multi-channel fluidized bed particle–supercritical carbon dioxide heat exchanger for concentrating solar applications

High-temperature thermal energy storage in oxide particles at temperatures above 600°C can couple concentrated solar energy with high-efficiency thermal power cycles to provide dispatchable solar-driven electricity. Challenges remain in developing cost-effective primary heat exchangers, which require expensive alloys, to extract the high-temperature thermal energy from the particles to power cycle fluids, such as supercritical CO2 (sCO2) in recuperated Brayton cycles. To explore one pathway for cost-effective, high-temperature particle heat exchangers, the current study demonstrates a shell-and-plate, particle–sCO2 heat exchanger with narrow-channel fluidized beds coupled with micro-channel sCO2 flows in the heat exchanger walls. This study evaluates the feasibility of multiple parallel, narrow-channel fluidized beds in shell-and-plate particle–sCO2 HXs, to achieve high bed-wall heat fluxes at elevated temperatures. A reduced-order model simulates the narrow-channel, fluidized-bed particle–sCO2 heat exchanger to design the fluidized bed geometry, in terms of depth, height, and number of channels,for a nominal 40-kWth heat exchanger at particle and sCO2 inlet temperatures up to 600°C and 400°C respectively. The resulting shell-and-plate heat exchanger design operates with bubbling fluidization of the downward-flowing oxide particles to enhance bed-wall heat transfer. The heat exchanger core is fabricated with etched sCO2 micro-channels in thin wall plates that are diffusion bonded to spacer frames to form the shell-and-plate structure with 12 parallel, fluidized bed channels, 10.4 mm deep. The heat exchanger is tested at the National Solar Thermal Test Facility at Sandia National Laboratories with CARBOBEAD HSP particles at design particle flow rates of 0.20 kg s−1 and inlet temperatures up to 525°C. Results show that fluidization across multiple parallel channel beds can maintain uniform particle inventory with a common freeboard zone above the heat exchanger core. Bubbling fluidization improves particle–wall heat transfer coefficients but also increases axial dispersion of particle thermal energy, which lowers the log-mean temperature difference such that total heat transfer remains relatively constant to within ±10% over a broad range of fluidization gas velocities. The axial dispersion required particle and sCO2 flow rates to be increased by 25% over model-designed conditions to achieve the targeted 40 kWth, which indicates the importance of incorporating axial dispersion into heat exchanger design models and of deploying bed structures to suppress it. This study demonstrates the feasibility and preferred fluidizing gas conditions for particle heat exchangers for releasing high-temperature thermal energy storage systems.

Evaluation of energy-saving potential and indoor thermal environment of passive solar houses in various heating climate regions of China

ABSTRACT Passive solar houses comprise crucial strategies of reducing heating and cooling energy for buildings, which have been extensively used in plenty of nations. This study develops a study on indoor thermal environment, the energy-saving performance and natural lighting in different heating climate regions (represented by Lhasa, Xining and Urumqi). An attached-sunspace solar house integrated with phase change material floors is also proposed to ameliorate indoor thermal comfort and simultaneously diminish the heating energy consumption. In addition, the dominant factors analysis of building envelope affecting building energy consumption are sensitively studied. Moreover, the influence of the depth of sunspace and the intensities of internal heat gain on indoor thermal conditions and heating energy consumption are further analyzed. The results indicate that among the three types of solar houses, the passive solar sunspace offers the most stable indoor temperature and Trombe-wall solar houses and attached-sunspace solar houses have striking energy-saving advantages compared with the traditional houses. And the annual power consumption of 695.1 kWh, 427.4 kWh and 688.8 kWh in Lhasa, Xining and Urumqi can be saved in view of natural lighting respectively. Additionally, the whole natural temperature of attached-sunspace solar house is on the rise apparently when the PCM floors are embedded in the house and the energy saving fraction of these three regions is 81.9% for Lhasa, 68.8% for Xining, 55.7% for Urumqi respectively with the integration of phase change floors. The sensitivity analysis of key parameters impacting building energy consumption from high to low are the heat transfer coefficient of external wall, the ratio of window to wall, air tightness, the heat transfer coefficient of external window, and the heat transfer coefficient of roof. The depth of 1.2 m for sunspace is recommended as the optimal choice. Indoor natural temperature is on the rise as intensities of internal heat gain increase. The same downward trend for the main bedroom and the whole building is displayed with the increasing interior heat sources in terms of annual heating load.

Optimization of splash lubrication in the gearbox considering heat transfer performance

Despite recognizing the importance of cooling within gear systems, there is a lack of clear design guidelines to minimize churning losses while enhancing heat dissipation performance. This study proposes a reliable approach to modeling using a steady-state approximation methodology based on heat-flow coupling for multiphysics around shrouded gears. Computational analyses reveal a distinctive multiphase flow induced by pressure gradients. The efficacy of the proposed methodology is empirically verified through its ability to characterize lubricant distribution and churning losses in splash lubrication systems. Furthermore, an innovative shroud optimization framework is introduced to regulate friction and collision between fluid particles and gear surfaces, enhancing splash lubrication performance. A genetic algorithm is employed within this framework to obtain shroud configurations for gears with a higher wall heat transfer coefficient (WHTC) and lower resistance torque. Implementing the optimal shroud design achieves a trade-off in terms of a notable 13.7% reduction in resistance torque alongside 11.6% and 4% increments in WHTC for the tooth root face and tooth face, respectively. This signifies enhanced reliability in multiphysics governing gearbox functionality, thereby providing valuable guidance for the design of shrouded gear systems.

Optimization strategies of the envelope insulation for a detached house based on load sensitivity and thermal storage performance

Reasonable thermal insulation in cold regions is the key to improve the indoor thermal environment. In this paper, the detached house is taken as the research object, and the sensitivity analysis method is used to quantify the influence of each parameter on the building heat load in three different climate zones. The attenuation characteristics of the heat storage body to the outdoor temperature wave are studied by using the A·M Shklovel calculation method, and the thermal insulation strategy of the envelope structure is optimized by genetic algorithm (GA). The results show that the heat transfer coefficient of roof and exterior wall has the most significant influence on the building heat load. The mean effect response of each factor shows that the Delta (Delta is the value used in Taguchi design methodology to express the relative effect of each factor on the response) of roofs in the three regions is the highest, 3.061, 4.061, and 5.88, respectively. The influence of the type and thickness of the insulation material on the heat storage performance is different. The indoor and outdoor temperature wave penetration attenuation multiple increases with the increase of the thickness of the insulation layer, increases with the decrease of the thermal conductivity of the insulation material, and increases with the increase of the specific heat capacity. The choice of insulation materials is not only related to the above two parameters, but also directly affected by the price. Considering the influence of various factors, the economy of choosing expanded polystyrene board for thermal insulation in the three regions is the best. The optimal thermal insulation thickness of the north wall and roof is 8 and 16 cm (3A climate zone), 10 and 17 cm (2B climate zone), 13 and 20 cm (2A climate zone), respectively.

Numerical Investigations of the Cooling Performance of an R410A Closed-Loop Spray Cooling System

To investigate the spray cooling characteristics and the impact of spray parameters such as chamber pressure, spray height, and spray tilt angle on heat transfer efficiency, a mathematical model based on the Eulerian–Lagrangian frame was established for an R410A closed-loop spray cooling system. The results revealed that the spray pattern is conical, with the center velocity significantly higher than the edge velocity. The temperature distribution of the cooling surface and liquid film height both exhibit a “W” shape, and the surface temperature is lower where the liquid film is thin. There is an optimal liquid film height of approximately 5 μm, at which the cooling surface temperature is the lowest. The surface temperature increases with an increase in the spray chamber pressure. Considering average cooling surface temperature, the optimal tilt angle is 40° with an average surface temperature of 330.1 K. When considering wall temperature and wall heat transfer coefficient uniformity, however, the optimal tilt angle is 10°, leading to the average surface temperature of 332.6 K. When increasing the optimal spray height to 70 mm, the average surface temperature is 313.4 K.

Influence of convective heat transfer and wall thermal capacity on dynamic interactions between wall temperature and pressure drop oscillations during microchannel flow boiling

Flow boiling in microchannel heat sinks is attractive for use in industrial thermal management applications owing to highly effective heat dissipation and compactness. However, flow boiling is subject to two-phase flow instabilities, specifically pressure drop oscillations, that may deteriorate performance by causing transient wall temperature oscillations or initiating premature critical heat flux. While there has been extensive model-based investigation of the stability and flow dynamics associated with pressure drop oscillations in microchannels, prior work has not focused on the coupled interactions between these flow oscillations and the dynamic heat transfer processes, as is required to predict thermal performance degradation and ensure reliable operation. This study predicts the influence of wall thermal capacity and heat transfer coefficient on the dynamic interaction between pressure drop oscillations and wall temperature. First, we demonstrate and explain how added wall thermal capacity acts to reduce the amplitude of pressure drop oscillations due to coupling between the thermal and flow dynamics. Whereas previous work has attributed this effect to a thermal time constant of the wall, the amplitude of the pressure drop oscillations is shown to scale with the Biot number by scanning various combinations of wall thermal capacity and heat transfer coefficient. A Biot number threshold is thereby suggested as the criterion for judging whether the thermal wall capacity must be considered in dynamic models, based on the strength coupling between the wall temperature and pressure drop oscillations.

Optimization of air-conditioning load for a residential building using Taguchi method

Economic growth has been raised in many countries in recent years, prompting people to seek out human comforts. For human comfort, residential buildings consume a significant amount of energy for air-conditioning systems. Building materials are the most significant factor in deciding the air-conditioning load. In this research work, a typical residential flat model is prepared using AutoCAD software and the building energy simulation is done for this model using hourly analysis program software. The results obtained from the building simulation are optimized using the Taguchi method. For optimizing the air-conditioning load, five influencing factors such as the overall heat transfer coefficient of wall (A), thickness of roof insulation material (B), heat transfer coefficient of window glass (C), bypass factor (D) and cooling coil supply temperature (E) of air-conditioning equipment at three levels are selected. For the experimental tests, an orthogonal L27 arrangement is chosen for experimental trials and in every trial, air-conditioning load of the typical residential flat has been calculated. After that, using Minitab 17 statistical software, the signal-to-noise ratios are obtained for different factors by selecting smaller-is-better option. After studying the response table for Taguchi tests, the optimum levels for all five factors are obtained. The analysis of variance method was also used to identify the main factors affecting the results significantly by finding out the contribution of all five factors in percentage. The results indicated that the most efficient set of all five factors is A1B3C1D2E3 and the most affecting factor is the wall material of the residential flat having a contribution of 91.91%.