Wall Heat Transfer Coefficient Research Articles

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%.

Computational fluid dynamics analysis of PCM-based ocean thermal engine with external rib turbulators

The concept of extracting energy from ocean thermal gradients for the long-term deployment of underwater profiling vehicles is promising. One potential method to capture ocean thermal energy is leveraging the volume change of phase-change materials (PCMs). However, the low thermal conductivity of PCMs and the low heat transfer coefficient between heat exchangers and seawater limit the performance of PCM-based ocean thermal engines (OTEngs). In this paper, a new type of heat exchanger structure with external rib turbulators is proposed. Detailed computational fluid dynamics analysis is conducted for the external forced convection heat transfer process of the heat exchanger with Reynolds number in the range of 240,000 to 600,000. In addition, a solver for calculating the phase-change process of the PCM inside the heat exchanger was developed based on OpenFOAM. The influence of the ribs on the heat transfer and energy conversion processes of the OTEng was analyzed. Results show that (i) the heat transfer coefficient of the ribbed heat-exchanger wall significantly improves, especially at low Reynolds numbers; (ii) 2.5–5 mm high ribs can effectively enhance heat transfer through the wall. When rib weight exceeds 5 mm, the heat transfer coefficient does not increase with the height; (iii) increasing the rib spacing reduces the water resistance and heat transfer coefficient of the wall; and (iv) the ribbed OTEng has a higher net power output compared with the conventional engine.

Optimization of air-conditioning load due to different thermal parameters for a residential building using different techniques

Better insulation or material with low thermal conductivity is a crucial factor in new construction and retrofitting existing structures where energy efficiency is the objective. Effective energy savings need the careful selection of construction materials to accomplish this. For this purpose, in this research work, a typical residential flat is taken into account. Using the hourly analysis program (HAP 4.90) software, the cooling and heating load is determined. Eight influencing factors, including building orientation (A), overall heat transfer coefficient of wall (B), roof insulation material (C), roof insulation thickness (D), heat transfer coefficient of glass (E), window to wall ratio (F), air infiltration flow rate (G), and bypass factor of air-conditioner (H) at three levels, are chosen to optimize the cooling and heating load. An orthogonal L27 layout is considered for the optimization, and the cooling and heating load has been determined for each experimental attempt. At first using Taguchi method, ANOVA analysis is done separately for each response variable and later using GRA and TOPSIS based Taguchi analysis, multi-objective optimization has been done, and the results are compared with each other. By determining each factor’s percentage contribution to the cooling and heating load, the ANOVA approach is also used to identify the primary components that have a major impact on the cooling and heating load. The outcome demonstrates that A3B2C2D3E1F1G1H1 is the best possible combination of all eight variables, and overall heat transfer coefficient of wall is the most influencing parameter for the residential buildings.

The influence of asymmetric truncated ribs on flow and heat transfer of supercritical aviation kerosene in a rectangular curved cooling channel

Coke deposition and heat transfer deterioration in curved cooling channels have drawn increasing attention. In this work, the influence of asymmetric truncated micro-ribs on flow and heat transfer in a curved cooling channel is numerically investigated. Three-dimensional numerical model is established for the reacting flow of aviation kerosene, RP-3 in both the smooth and ribbed cooling channels. The results show that compared with the non-ribbed and the top heated wall-ribbed channels, the bottom heated wall-ribbed channel renders lower temperature and less coke deposition on the top wall with more uniform temperature field in the channel. Heat transfer deterioration at the top heated wall is significantly improved in the bottom heated wall-ribbed channel. As rib number of the bottom heated wall-ribbed channel increases from 0 to 18, the velocity of the secondary flow in curved channel increases by 68.7 %, heat transfer coefficient of the top wall rises by 50.6 % while coke deposition on top wall is reduced by 34.9 %, respectively. With the height of ribs in the bottom heated wall-ribbed channel increasing from 0 mm to 0.4 mm, temperature and coke deposition on the top heated wall decreases by up to 300 K and 59.26 %, respectively. Meanwhile, heat transfer coefficient at the top wall increases by as much as 69.23 %. The interaction between vortex structures and the micro-ribs enhances heat near the heating wall surface. Additionally, coke formation is inhibited due to enhanced secondary flow in the ribbed channel. However, pressure drop of the cooling channel rises rapidly with the increase of height and number of ribs. Considering both the overall thermal performance and flow resistance of the cooling channel, the bottom heated wall-ribbed curved channel with rib number of 9 and rib height of 0.2 mm is recommended in the present study.

Wall friction and heat transfer in turbulent pulsating flow in a square duct

For the first time, a DNS study of pulsating turbulent flows and heat transfer in a square duct is performed. Calculations were carried out at Re=2200 in the current-dominance mode for eight values of the oscillation period and three values of the Prandtl number. It was found that the time-mean friction coefficient in the studied pulsating flows has a lower value than in the stationary flow. The difference reaches −14.7%. The values of the Nusselt numbers are also decreasing, albeit by a smaller amount. The features of secondary flows of Prandtl’s 2nd kind at different phases of the flow oscillations are studied. The profiles of the wall friction stress and heat transfer coefficients and their evolution over time are determined. The time-oscillations of the wall friction are entirely determined by the oscillations of the main flow, being ahead of it in phase by π/4, which coincides with the behavior of laminar pulsating flows at high frequencies. The oscillations in heat transfer on the wall are determined by oscillations in the intensity of turbulent heat exchange, and the change in the latter occurs almost in phase with a change in the volume-average intensity of the transverse velocity fluctuations.

Effects of heating configuration and operating parameters on heat transfer and interfacial physics of microgravity flow boiling with subcooled inlet conditions – Experiments onboard the International Space Station

This study is part of the Flow Boiling and Condensation Experiment (FBCE), a collaborative effort between the Purdue University Boiling and Two-Phase Flow Laboratory (PU-BTPFL) and the NASA Glenn Research Center. The FBCE fitted with the Flow Boiling Module (FBM) was launched to the International Space Station (ISS) in August 2021 and experiments were successfully performed from February to July 2022 to amass a large microgravity-flow-boiling database. This study is focused on heat transfer and flow visualization of microgravity flow boiling of n-Perfluorohexane in a rectangular channel of 5.0 mm height, 2.5 mm width (heated), and 114.6 mm length, with subcooled inlet conditions. High-speed-video photography is utilized to present flow patterns and temporal interfacial behavior. Heat transfer results are presented in the form of flow boiling curves and both parametric curves and streamwise profiles of wall temperature and heat transfer coefficient. Firstly, the parametric effects of mass velocity (199.4 – 3200.0 kg/m2s), inlet subcooling (0.2 – 46.0°C), and inlet pressure (124.2 – 176.7 kPa), on the aforementioned aspects are assessed for double-sided heating to establish them for a microgravity environment. Of these three parameters, mass velocity and inlet subcooling mostly determine the microgravity flow boiling behavior, while inlet pressure plays an insignificant role. Flow patterns for double-sided heating are more complex than those for single-sided heating due to interaction between the two vapor layers. Vapor interaction is minimized at high subcoolings and high mass velocities due to strong condensation offered by the subcooled bulk liquid layer separating them. Despite the different flow patterns, both single- and double-sided heating generally result in similar parametric trends and local heat transfer coefficients for similar operating conditions. Flow instabilities manifest as temporal flow anomalies and temperature oscillations, and their severity increases with increasing boiling number. Secondly, the effects of heating configuration are analyzed by comparing and contrasting several aspects of single- and double-sided heating data. The heat fluxes at which onset of nucleate boiling degradation (ONBD) and critical heat flux (CHF) occur are distinctly different for single- and double-sided heating. There exists a threshold inlet subcooling demarcating the dominance of flow acceleration and condensation effects in vapor removal from the near-wall region and replenishment of fresh liquid for boiling. Above the threshold, condensation from the near-wall region is dominant and single-sided heating yields higher heat fluxes, and below it, acceleration is dominant and double-sided yields higher heat fluxes. At mass velocity in the range of 200 – 2400 kg/m2s, the threshold inlet subcooling lies in the approximate range of 20 – 30°C (corresponding inlet quality of roughly -0.40 – -0.20).

Construction of Building an Energy Saving Optimization Model Based on Genetic Algorithm

The building envelope structure is the cause of a major part of energy consumption, with exterior walls and windows being the main energy-consuming components. Traditional building energy conservation measures often overlook the demand for human comfort, especially in areas characterized by hot summers and warm winters. For this paper, the authors concentrated on indoor comfort, with a focus on optimizing the heat transfer coefficient of windows and exterior walls using a genetic algorithm. They used a genetic algorithm to explore the performance optimization of exterior walls and windows in the enclosure structure. To aid this effort, they constructed a building energy-saving optimization model. In addition, they created an optimization model in an attempt to reduce building energy consumption. They took the heat transfer coefficients of the outer window and the outer wall as the optimization parameters of the established model, and they compiled the optimization program using MATLAB. The experimental results showed that the heat transfer coefficients of exterior walls and windows in cold regions are 0.4459 and 2.7875, respectively, while the heat transfer coefficients in warm winter and hot summer regions are 0.66, 1.98, 1.026, and 1.59. The conducted work provides a reference for the optimization design of the heat transfer coefficient of external walls and windows as a measure to enhance building energy efficiency.

Influences of Stefan blowing on unsteady flow of Casson nanofluid past a stretching surface

This paper deals with the flow of non-Newtonian nanoliquid past an unsteady stretched exterior in attendance of Stefan blowing/suction. Model for Casson liquid has been taken into account to represent the non-Newtonian liquid. Two phase fluid replica for nanofluid has been considered. Similarity alterations are applied to transform the leading pdes (partial differential equations) to ordinary ones. Runge–Kutta method of order four has been used to solve the altered equations numerically with the help of shooting technique. Wall shear stress, heat and mass transfer coefficients diminish with the rise in unsteadiness. Fluid's velocity and temperature are found to rise with the enhancement in Stefan blowing/suction parameter. Temperature of the fluid rises but fluid's velocity diminishes for the rising values of Casson parameter. Basically, the pertinent parameters affect the stream, warmth and accumulation transport significantly which can be viewed by the figures and tables presented here and also by their physical analysis.

Development of a New Heat Transfer Correlation for Flow of Supercritical Steam in Spiral Tubes Under Deteriorated Heat Transfer Regime

Abstract Heat transfer to supercritical steam flowing inside spiral tube under deteriorated heat transfer regime is investigated in this work using three-dimensional numerical simulations. The numerical solver is first validated with experimental results and subsequently multiple simulation cases are formulated with spiral tube by varying the flow and thermal conditions. The wall temperature and heat transfer coefficient of spiral tubes are compared with the same of straight tubes. Over the parameters range considered, the heat transfer coefficient is improved by 88% when straight tube is replaced with spiral tube, for similar flow and thermal conditions. Most significantly, the deteriorated heat transfer regime, which is otherwise witnessed in straight tubes is completely avoided by using spiral tubes, for the similar flow conditions. Tangential velocity, which arise due to centrifugal force is the primary reason of enhanced heat transfer in spiral tubes. Three standard heat transfer correlations are tested for spiral geometry under deteriorated heat transfer regime and it is found that none of those can predict the heat transfer coefficient accurately. Using the simulation results along with Buckingham π-theorem, a new correlation for heat transfer coefficient is proposed for spiral tubes carrying supercritical steam and operating under deteriorated heat transfer regime. The new correlation can predict the wall temperature and heat transfer coefficient accurately. The correlation has an error band of ±25%.

The effect of thermal insulation from autoclaved aerated concrete on the energy performance of a single-family house

The article examines the effect of thermal insulation from autoclaved aerated concrete on the energy characteristics of a single-family house. Analysis of mathematical models of energy characteristics of external enclosing structures of buildings depending on the thickness of AEROC autoclaved concrete products according to the criteria of heat loss shows that the thermal resistance value of 7.11 m2K/W and the heat transfer coefficient of 0.141 W/(m2K) are achieved using the wall block AEROC D 300 and AEROC Energy thermal insulation panel with a thickness of 200 mm. These indicators correspond to the passive house standard for thermal resistance (Ro ≥ 6.7 m2K/W) and heat transfer coefficient (Uo ≤ 0.15 W/m2K) of external walls, which confirms the feasibility of using AEROC Energy D 150 thermal insulation panels in combination with wall blocks AEROC D 300 for housing construction according to passive construction standards.

Numerical Investigation on Pool Boiling Heat Transfer of Silica and Alumina Nanofluids

Surface modification through pool boiling of nanofluid is an efficient technique in delaying critical heat flux and attaining high heat flux at lower wall superheat temperature. The method of involving phase interaction, bubble evolving and dynamics phenomenon, and distribution of vapor void fraction altogether is the motivation behind current topic. The present work will contribute to the recovery and reuse of the wastewater and incinerator heat, cooling of high heat flux generating micro-electronics chips, and cooling of fast breeder test reactors. In the present study, the nucleate pool boiling of SiO2/water and Al2O3/water nanofluid are studied numerically and the effect of wall superheat temperature and heat flux on active nucleation site density, bubble departure diameter, and bubble waiting time are studied. The effect of various heater surface material and nanofluid concentration on wall superheat temperature and heat transfer coefficient are also investigated. A two-phase Eulerian approach, heat flux wall-partitioning model, and Rensselaer Polytechnic Institute boiling model are considered to simulate the current model. The pool boiling of nanofluids resulted in the formation of nano-porous layer on heater surface because of microlayer evaporation process that enhanced the heat transfer coefficient and reduced the wall superheat temperature.

Convective heat transfer coefficient for the side‐wall in a fixed bed

AbstractFixed beds are widely used in the chemical and process industry due to their relatively simple yet effective performance. Determining the radial heat transfer at the wall in a fixed bed is crucial to predict the performance of columns. Heat transfer parameters often need to be obtained experimentally. Various Nusselt versus Reynolds correlations in literature show considerable scatter and discrepancies. The tube‐to‐particle diameter ratio and boundary conditions on the particle surface have been understood to affect heat transfer near the wall by virtue of influence on the near‐wall porosity and mixing. In this work, a fixed bed consisting of mono‐disperse particles is generated via gravity‐forced sedimentation modelling utilizing the discrete element method for a ratio of 3.3. The system is meshed and imported in a computational fluid dynamics (CFD) solver. Fluid inlet velocity is varied to get corresponding to the laminar and turbulent flow regimes. The particles are treated as boundaries with Dirichlet, Neumann, and Robin boundary conditions applied for the closure of energy balance. Another set of simulations is run with particles modelled as solids with varying thermal conductivities (). The heat flux and volume‐averaged fluid temperature calculated during post‐processing are used to determine the wall heat transfer coefficient and, subsequently, the wall Nu number. Fifteen versus correlations are compiled and analyzed. A new semi‐empirical correlation for the wall Nusselt number has been developed for a fixed bed packed with monodisperse spheres for and results compared with data published in literature. Additionally, the impact of buoyancy effect on the wall Nusselt number has been studied.