Wall Heat Transfer Coefficient Research Articles

Condensation Heat Transfer of R-407C in Helical Coiled Tube Heat Exchanger

The results of an experimental study to evaluate the characteristics of R-407C thermofluid during condensation in a helically coiled copper tube heat exchanger are presented. The effects of saturation temperature (Tsat), and mass and heat fluxes of refrigerant R-407C on thermal performance and pressure drop were determined. The relationship between the refrigerant wall subcooling and heat transfer coefficients was also investigated. This paper reports the effect of the temperature of the water used as cooling medium on the heat transfer rate of condensing R-407C. The study was conducted with mass flux of R-407C in the range of 100–450 kg/m2s, mass flux of the coolant water in the range of 500–5000 kg/m2s and Tsat of 31 °C, 35 °C, and 39 °C. Compared with a straight smooth tube, the use of the helical coiled (helicoidal) tube increased the condensation rate with a corresponding pressure drop that depended on the value of Tsat of the refrigerant and temperature of the coolant.

Research on Energy-Saving Design of Rural Building Wall in Qinba Mountains Based on Uniform Radiation Field

How to create a healthy and comfortable indoor environment without causing a substantial increase in energy consumption has become a strategic problem that the development of all countries must face and solve. According to the climatic conditions of Qinba Mountains in China, combined with the characteristics of local rural residential buildings and residents’ living habits, the field survey and theoretical analysis were used to study the thermal environment status and the heating energy consumption condition of local rural residential buildings. The thermal design method of walls for the local rural energy-saving buildings based on the indoor uniform radiation field was explored by using the outdoor comprehensive temperature function expressed by the fourth-order harmonic Fourier series as the boundary condition of the wall thermal analysis. ANSYS CFX was adopted to study the suitability of the energy-saving wall structure designed by the above method. The results show that the indoor thermal environment of local rural residential buildings in winter is not ideal and the heating energy consumption is high, but this area has the geographical advantage to develop solar energy buildings. It is proposed that the indoor thermal comfort temperature of local rural residential buildings in winter should not be lower than 14°C. When the internal surface temperature of the external walls in different orientations are equally based on the design principle of uniform radiation field, the heat transfer coefficient of the east wall, the west wall, and the north wall of the local rural residential buildings is 1.13 times, 1.06 times, and 1.14 times of the south wall heat transfer coefficient, respectively. The energy-saving structural wall with KPI porous brick as the main material and the south wall heat transfer coefficient of 0.9 W/(m2·K) is the most suitable energy-saving wall for local rural residential buildings.

CFD investigation of biogas reformate using membrane-assisted water gas shift reaction: Parametric analyses

A detailed computational fluid dynamics (CFD) model has been developed in this study for the water gas shift of untreated biogas reformate in both fixed bed and membrane reactors to gain insights into the interaction of 3Ni5Cu/Ce0.5Zr0.33Ca0.17 catalyst with fluid transport phenomena taking place in the reactor. The developed CFD model closely agreed with data from both literature and experiments. An average absolute deviation of 8.59% and 6.32% was obtained when the CFD model was validated with a fixed bed reactor and membrane reactor experiments respectively. The membrane reactor enabled equilibrium conversion at 2.95gcat.h/mol CO weight-time with equilibrium conversion attained at lower weight-time and steam to carbon ratio of 3.4. Also, it was found that the counter-flow configuration resulted in better enhanced CO conversion, and the wall heat transfer coefficient above 8.6W/m2/K had negligible on conversion, amounting to only a 1% increase in conversion for both reactors.

MHD flow and melting heat transfer of dusty Casson fluid over a stretching sheet with Cattaneo–Christov heat flux model

ABSTRACT The features of magnetohydrodynamics (MHD) flow and melting heat transfer of dusty Casson fluid over a stretching sheet have been disclosed. The modified version of Fourier's law through Cattaneo–Christov heat flux is employed. The mathematical formulation is yielded in the form of ordinary differential equations. The attained coupled system has been scrutinised with the aid of Runge–Kutta–Fehlberg 45 order method along with shooting technique. In order to produce accuracy and refinement in the consequences, the coefficients of Runge–Kutta scheme are obliged. To get complete intellect about the current pagination, the impact of flow controlling variables on pertinent profiles are evaluated in a quantitative and qualitative manner. The computations of wall friction factor and heat transfer coefficient are presented in graphical and tabular forms. It is noticed that, due to the presence of the melting effect, the temperature and thermal boundary layer thickness are higher for relaxation time parameter.

Numerical investigation on convective heat transfer of supercritical aviation kerosene in a horizontal tube under hyper gravity conditions

The present study numerically investigates the convective heat transfer characteristics of supercritical aviation kerosene in a horizontal tube under hyper gravity conditions with various gravitational accelerations. Effects of mass flow rate (1.5 g/s∼2.5 g/s) and heat flux (500 kW/m∼2700 kW/m2) on heat transfer under hyper gravity conditions are discussed. Analyses focused on the influences of buoyancy force on heat transfer characteristics of the top and bottom wall under hyper gravity conditions. The model is simplified as a horizontal tube with the diameter of 1.8 mm and the length of 250 mm. The results show that the gravity plays a dominant role and the secondary flow structure is induced due to the drastic decrease in density in the pseudo-critical region. Two types of deterioration occur when the wall temperature and bulk temperature reaches the pseudo-critical point respectively. The onset of the deterioration occurs earlier with the increase of gravitational acceleration. As the gravitational acceleration increases, the difference between the top and bottom wall heat transfer coefficient (HTC) becomes larger. The criterion Grq/Grth<1 proposed by Petukhov is proved to be applicable to evaluate the effects of buoyancy.

Winter Thermal Environment and Thermal Performance of Rural Elderly Housing in Severe Cold Regions of China

Understanding the thermal performance of the residential envelope is important for optimizing the indoor thermal environment. In this study, the indoor thermal environment and thermal performance of rural residences housing the elderly was determined through field measurements in Qiqihar in 2017 and 2019. The results revealed that the living room temperatures in more than 50% of homes were below the thermal neutral temperature for the elderly (17.32 °C). Moreover, the indoor thermal environment changed significantly during the day, with the predicted mean vote during the day fluctuating from 2 to 4 units. The air change rate of living rooms in 2017 and 2019 was 0.20–2.20 h−1 and 0.15–1.74 h−1, respectively. Residential ventilation times detected by an air-tightness detector ranged from 0.40–1.49 h−1. Furthermore, infrared thermography (IRT) detected air leakage in the windows of the all houses in this study, as well as thermal bridges and condensation on the exterior walls of several houses. The heat transfer coefficient of the exterior walls of all houses detected by IRT was 0.25–0.74 W/(m2·K), and a significant positive correlation was observed between the heat transfer coefficient of the south wall and the window-to-wall ratio. Finally, the heat transfer coefficient of the external walls exhibited a negative but not significant correlation with indoor temperature. This study provides detailed data and guidance for improving the indoor environment of rural houses in severe cold regions.

Study Influencing Factors for Energy Consumption of Office Buildings in Severe Cold and Cold Region, China

The paper analyzed and studied the factors that affect the energy consumption of office buildings, and some suggests were put forward according to the analysis. Through orthogonal experiment the research selected the exterior wall heat transfer coefficient, exterior window heat transfer coefficient, lighting power density, electricity density, air conditioning system form to analyze and study. Although the ranking factors affecting two models are different, but through the analysis found that the air conditioning system is the greatest impact factor, followed by the lighting equipment, after that is envelope.

Effects of inlet turbulence intensity on wall heat transfer in a turbine guide vane

Heat transfer is an important phenomenon that exists in many industrial applications, especially for gas turbines, aeronautical engines. In this work, two different turbulence models ([Formula: see text] and SAS model) are used to investigate the effects of inlet turbulence on wall heat transfer and the characteristics of flow field in a well-known turbine guide vane (LS89). In order to handle the transition, Menter’s [Formula: see text] transition model is used. The simulations show that the inlet turbulence has an apparent effect on the wall heat transfer of the vane. Not only the maximum wall heat transfer coefficient is increased, the distribution of wall heat flux at the suction side is also modified. The isentropic Mach number along the vane surface is insensitive to the variance of inlet turbulence intensity. Besides, a shock appears in the throat and a laminar-to-turbulence transition position moves forward after the main flow turbulence is enhanced. Moreover, the results indicate that SAS model is capable of capturing more flow structures such as reflecting pressure waves and shedding vortexes while the [Formula: see text] model misses them due to the dissipation.

Wall heat transfer in the inverted annular film boiling regime

Inverted annular film boiling (IAFB) is one of the post critical heat flux (post-CHF) regimes, which may occur during the core reflooding phase following large-break loss of coolant accidents in water-cooled nuclear reactors. This paper first analyzes wall heat transfer characteristics and summarizes convective wall heat transfer coefficient correlations developed for the IAFB regime in the literature. Parametric effects of the inlet subcooling, mass flux, and pressure on the wall Nusselt number in the IAFB regime are studied. It is found based on the experimental data in the literature that the system pressure has negligible effect on the wall Nusselt number while the effects of the mass flux and inlet subcooling are complex and depend on the flow conditions. The modified Bromley model and laminar flow model with an enhancement factor for the IAFB regime, currently used in COBRA-TF and TRACE, respectively, are compared with Stewart’s experimental data under subcooled and low-quality conditions in the IAFB regime. Both of these two models under-predict the heat transfer coefficients calculated from Stewart’s experimental data for high flow and high subcooling conditions. Based on the analysis of the parametric effects, two new wall heat transfer correlations are proposed by correlating the Nusselt number and non-dimensional vapor film thickness using Stewart’s experimental data and benchmarked against the experimental data in the literature. These two new correlations cover ranges of the system pressure from 2.0 to 9.0 MPa, mass flux from 215 to 923 kg/m2-s, and inlet subcooling up to 50 °C.

Investigation on the heat transfer coefficient and uniformity in a polygonal furnace of a 350 MW supercritical CFB boiler

The first international 350 MW supercritical circulating fluidized bed (CFB) boiler with a polygonal furnace was put into operation successfully in February 2018. The temperature profiles of the furnace water walls on four different heights, the temperature profiles near the outlets of the furnace water walls and dual-side water walls were measured under boiler loads of 50%, 75% and 100% BMCR, respectively. The coefficient and uniformity of heat transfer were investigated according to the test results. The results indicate that the temperature deviation of the furnace water walls on each layer at four different heights is no more than 6 °C, which illustrates the reliable hydrodynamic safety of the water walls of the polygonal furnace. Under boiler load of 100% BMCR, the temperature deviations of the front, rear, left and right water walls near the outlets of water walls are no more than 37 °C, and the temperature deviations of the 11 dual-side water walls are no more than 31.1 °C. As the boiler load increase from 50% BMCR to 100% BMCR, the average heat transfer coefficient of water walls increases from 98.1 W/(m2·oC) to 154.2 W/(m2·oC).

Effects of multi-factors on comprehensive performance of a hydrogen-fueled micro-cylindrical combustor by combining grey relational analysis and analysis of variance

In order to address the issue about the impact ranking of multi-factors on the comprehensive performance of a hydrogen-fueled micro-cylindrical combustor, in this work, the Taguchi experimental design is adopted to arrange numerical cases, grey relational analysis is used to determine the impact ranking of multi-factors, and analysis of variance is used to validate the results of grey relational analysis and achieve the contribution of each factor. Besides, effects of weight matrixes of performance parameters (radiation power, radiation efficiency and combustion efficiency) are investigated. It is found that the influence ranking of factors on comprehensive performance of a hydrogen-fueled micro-cylindrical combustor is related with the gap between weights of prime performance parameters and weights of secondary performance parameters. The influence ranking is decided by the significance of three performance parameters. However, it is still found from the analysis of variance that hydrogen/air equivalence ratio (Factor A), wall emissivity (Factor E) and convective heat transfer coefficient of outer wall (Factor F) are the prime factors affecting the comprehensive performance of a hydrogen-fueled micro-cylindrical combustor. This work provides us significant reference for optimizing the micro-cylindrical combustor.

Comparison of Convection Models for Hot Blast Stove Regenerative Chamber

Low error numerical solutions of convective heat transfer in high-temperature air regenerative heat exchangers in the convective regime rely on the correct determination of the wall heat transfer coefficient. Presented are results of simulations computed using a similarity theory-based numerical model. They imply that the current empirical formulas based on the theory of similarity, derived for analogous model cases of convective heat transfer do not allow to achieve satisfactory accuracy. Actual dependencies of parameters and criteria yielding more representative heat transfer coefficient values might be obtained through the means of experimental research in thermodynamics of an experimental device.

Analysis of a 2-D model of a packed bed reactor for methanol production by means of CO2 hydrogenation

In recent years, methanol received the attention of many researchers as a building block of the circular economy, because of its diversified applications in different areas. Generally, methanol is produced by syngas, however recent studies are dealing with its production via carbon dioxide hydrogenation. With the aim to predict conversions, efficiencies as well as concentration, pressure and temperature profiles inside the packed bed methanol reactor, mathematical models are developed in one- (1-D) and two- (2-D) dimensions. However, a deep study about a 2-D mathematical model and conditions where its use is advisable to get reliable predictions is missing in the literature. In this research, a two dimensional model for methanol reactor via carbon dioxide hydrogenation is suggested, comparing a structured catalytic packing with a more common packed bed of catalyst pellets, which differ mainly for the respective thermal conductivity. The system of partial differential equations is solved in MATLAB® and the same operating conditions set in a previous work about a one dimensional model are considered. Results show that the 2-D model is useful for both reactor typologies under the examined operating conditions, although definitely more important for the non-structured reactor, where higher temperature and concentration differences on tube cross sections are calculated because of a stronger resistance to radial heat transfer. In addition, a higher efficiency is predicted for a structured reactor in terms of carbon dioxide selectivity to methanol and methanol yield, then a lower recycle flow rate is required in this case. A sensitivity analysis is also developed for the two reactor typologies, changing feed inlet temperature, wall heat transfer coefficient and tube diameter. Conditions are investigated, for which 2-D model results tend to corresponding outputs of a 1-D model.

Simulation on heat transfer performances of hollow tube electric heaters with dual-side longitudinal flow

PurposeConventional electric heaters mostly use U-shaped electric heating tubes and the hollow tube electric heaters are new type ones that rely on the heat transfer tubes as heating elements. However, in the original design, the fluid flows through the annular gaps between the shell wall and the supporting plates, the chambers between supporting plates are generally stagnant zones. The purpose of study is to overcome these deficiencies.Design/methodology/approachA modified approach is proposed in which the heating tubes are surrounded by holes on the supporting plates, thus the stagnant flow zone can be eliminated and the heating surfaces of both inside and outside the tube can be fully used. Numerical simulations were carried out on four schemes of hollow tube electric heaters, i.e. plate blocked, countercurrent, parallel and split. The results show that the two schemes of parallel and split can reduce the temperature difference between the two sides of the fixed tube plate, and thus reduce thermal stress and prolong the service life.FindingsThe split scheme of electric heater has the highest comprehensive index, moderate heat transfer coefficient and minimum pressure drop on the shell side. Its average heat transfer coefficient and comprehensive index are, respectively, 15.7% and 52.9% higher and its average pressure drop and tube wall temperature are, respectively, 57.6% and 19 K lower than those of the original plate blocked scheme, thus it can be recommended as the best scheme of the hollow tube electric heaters.Originality/valueBased on the original design of hollow tube electric heater with plate blocked scheme, three plate perforated schemes were proposed and investigated. The thermal and flow features of the four schemes were compared in terms of heat transfer coefficient, pressure drop and comprehensive index ho·Δpo−1/3. The split scheme can reduce the temperature difference between two sides of the fixed tube plate with reduced thermal stress. It has moderate tube wall temperature and heat transfer coefficient, the smallest shell side pressure drop and the highest comprehensive index ho·Δpo−1/3, and it can be recommended as the optimal scheme.

Analysis of Thermally Controlled Poultry Housing Using CFD

Poultry industry’s development in the past two decades and the need for increased animal protein sources in the hot regions of the world, require the need to develop housing system that is thermally controlled for optimal production. The research was carried out at Federal University of Technology Akure, Ondo State, Nigeria. The facility consisted of a broiler house of 6 rooms enclosed by masonry sidewalls at the base and insulated plywood at the upper section of the house with each experimental room equipped with blower, suction fan and heater. The data were monitored at the most critical time of the day – 1 pm during the dry season. Experimental data were recorded using developed and calibrated data logger. The 5 experimental rooms are programed to 5 temperature levels (41, 38, 35, 32 and 29°C) characterizing extreme heat boundary conditions for broilers with fans programmed at 1.5 m/s air velocity. The aim of this study is to evaluate the thermal distribution in solid-wall broiler houses using computational fluid dynamics (CFD). The CFD technique allows visualizing air flow according to different running condition for each room for exhaust fans, as well as other parameters. The simulation was used to determine the air temperature variation, inner wall temperature, external temperature, air velocity distribution, external wall heat flux, pressure and wall heat transfer coefficient in all the experimental rooms of poultry house. The simulated air flow pattern and temperature distribution in the experimental rooms were analyzed and the result revealed increase room temperature as the preset room temperature increases. However, the velocity profile in all the room shows buildup of air at the outlet vent due to turbulence created by the suction fans. The pressure profile across the rooms was relatively the same.

Experimental investigation on convection heat transfer of supercritical hydrocarbon fuel in a long mini tube

Thermal management system is crucial for hypersonic vehicles, in which a large amount of thermal energy due to aerodynamic heating needs to be removed by hydrocarbon fuel. In this study, an experimental investigation is performed on the convection heat transfer of supercritical hydrocarbon fuel in a long mini tube. The horizontal test tube has an inner diameter of 1.0 mm and a length-to-diameter ratio of 980, which are close to those of practical heat-transfer channels in the thermal management system for hypersonic vehicles. The non-linear distribution characteristics of wall temperature and heat transfer coefficient along the test tube are presented. The heat transfer regions of hydrocarbon fuel are classified into initial heat transfer (IHT), normal heat transfer (NHT), enhanced heat transfer (EHT) and deteriorated heat transfer (DHT). The heat transfer mechanisms in each heat transfer region are discussed in detail. The heat transfer of hydrocarbon fuel are affected by the dramatic variations in thermo-physical properties and buoyancy effect. Heat transfer is commonly deteriorated under disadvantageous thermo-physical properties and absence of buoyancy effect. The results could be beneficial for the analysis and design of the thermal management system for hypersonic vehicles.

Thermally stratified nanofluid flow over porous surface cone with Cattaneo–Christov heat flux approach and heat generation (or) absorption

This study deals with Cu-water nanofluid flow over porous surface cone in a thermally stratified medium. Heat transfer is examined with Cattaneo–Christov heat flux model instead of Fourier’s law along heat generation/absorption. Effects of free convection, magnetic field and suction are also reported. Transformations are utilized to attain constitutive laws of flow in form of ordinary differential equations, which are then dealt with Runge–Kutta–Fehlberg and shooting scheme. Physical impacts of parameters involved are discussed and presented graphically and through tabular values. Velocity enhances with Grashof number but not with magnetic parameter. Heat generation/absorption, thermal stratification and thermal relaxation parameter reduces temperature. The effect of heat generation/absorption and thermal relaxation parameter is to increase heat transfer. Results reveal that effects of thermal stratification parameter becomes less dominant to wall heat transfer coefficient with intensification in heat generation/absorption parameter.

Wall heat transfer coefficient and effective radial conductivity of ceramic foam catalyst supports

In this work, different tubular solid-foam packed-beds were experimentally investigated under air flow in order to determine the effective conductivity and wall-to-bed heat transfer coefficient. Several foam types with different material, namely silicon carbide and alumina, and cell density were tested. Foam samples were experimentally studied using different air flow rates and wall temperatures ranging between 200 and 500°C. Axial and radial steady-state temperatures in the bed were experimentally measured and analysed with a two-dimensional, one-equation model. A Chilton–Colburn-type correlation based on the reactor diameter as characteristic dimension was proposed for the wall-to-bed heat transfer coefficient, which proved good to describe the behaviour of all tested foams. Effective radial thermal conductivity and pressure drop in the foams were also characterized.

3-D computational investigation and experimental validation of effect of shear-lift on two-phase flow and heat transfer characteristics of highly subcooled flow boiling in vertical upflow

The present study is focused on development of a 3-D computational approach to predict highly subcooled nucleate flow boiling in vertical upflow. Investigation of existing computational methodology based on Volume of Fluid (VOF) approach revealed fundamental weaknesses in modeling multiphase flows, which stems mostly from inadequate representation of shear-lift force on bubbles. A user defined function is adopted which provides detailed information relating to this important effect, and computed results are validated through comparison with experimental results and analytic predictions of single bubble trajectory. The computational method is then used to model the entire flow field for subcooled flow boiling in a rectangular channel heated on two opposite walls, and predictions are validated against FC-72 experimental data for four different mass velocities. Overall, computationally predicted interfacial behavior, flow pattern, and heat transfer parameters (wall temperature and heat transfer coefficient) show good agreement with experimental data. The model is also shown highly effective at predicting local phenomena (velocity and temperature profiles) not easily determined through experiments.

Effects of particle contact characteristics on heat transfer performance of calcined petroleum coke heat exchanger

The present paper investigated the effect of particle contact characteristics (contact area coefficient) on heat transfer performance of heat exchanger. An unsteady heat transfer model of single-layer calcined petroleum coke was established. The average temperature, heat storage, heat transfer contribution rate and wall heat transfer coefficient were studied with different times. The research shows that with increase of contact area coefficient, the heat conduction capacity is enhanced and the average temperature of particles layer reduction rate and the heat storage reduction rate are obviously accelerated. The average contribution rate of solid phase heat transfer, wall comprehensive heat transfer coefficient and heat release rate increase, and the effective heat transfer time decreases. When the contact area coefficient increased from 0.03 to 0.07, the average contribution rate of solid phase heat transfer increased from 83.4% to 90.5%. The wall heat transfer coefficient increased from 12.7 W/(m2·K) to 27.1 W/(m2·K), the heat release rate increased from 83.79% to 98.2%, and the effective heat transfer time decreased from 8.02 h to 5.07 h.