Determination Of Heat Transfer Coefficient Research Articles

Effect of temperature sampling methods on sphere packed tube heat transfer coefficient determination

This paper investigates the heat transfer characteristics and the pressure drop of a sphere packed tube using Computational Fluid Dynamics (CFD) simulation. The turbulent kinetic energy of the fluid exhibits significant variation based on the differing accumulation structures of the particles, which is deemed to greatly influence the heat transfer characteristics. Consequently, the temperature distribution at the outer wall becomes inhomogeneous. Previous studies have overlooked the relative positioning between the thermocouples and the particles, potentially leading to inaccurate estimations of the heat transfer coefficient and discrepancies between CFD simulations and real-world experiments. To address this, two temperature sampling methods are employed in this paper to estimate the heat transfer coefficients: one method involves measuring the temperatures at various sampling positions, while the other uses the surface average temperature of the packed area as the sample temperature. Interestingly, it is found that different temperature sampling methods significantly impact the heat transfer coefficients. This study serves as a reminder for future research on sphere packed tubes to prioritize temperature sampling positions.

Metallurgical Method of Determining Heat Transfer Coefficient in Simulations of Twin-Roll Casting

We herein suggest a metallurgical method using pure aluminum with no freezing temperature range to derive appropriate roll/melt interfacial heat transfer coefficients in simulation of twin-roll casting process. This method is inspired by the concept that the position of the kiss points where two solidifying shells encounter and the roll nip coincides under the condition where the roll load becomes zero as the roll rotation speed decreases. The conditions where the roll load becomes zero under various melt supply temperature conditions in the actual TRC process are found experimentally. These conditions are then applied to simulation models to derive heat transfer coefficient values. When comparing these values with those derived previously from the empirical relation for roll rotation speed and heat transfer coefficient, the conclusion is drawn that the deviation was reasonably low, around 10%.

Determination of interfacial heat transfer coefficient at the frozen sand mold casting process of ZL101 alloy

Compared to the resin sand mold casting process, frozen casting is more environmentally friendly, providing a better working environment and enhanced supercooling degree. The interfacial heat transfer coefficient (IHTC) between frozen sand mold and metal is an important parameter that significantly influences the final mechanical properties and microstructure of the castings. This paper solved the inverse heat conduction problem using the finite difference method (FDM). In addition, the conjugate gradient method (CGM) was adopted to calculate the temperature distribution and heat flux in the molten metal. At the same time, the particle swarm optimization algorithm (PSO) was used in temperature distribution determination in frozen sand mold. The interfacial heat transfer coefficient (IHTC) was estimated during the solidification of ZL101. The results showed a good agreement between calculated and experimental data, obtaining accurate casting interface temperature Tm, frozen sand mold interface temperature Ts, heat flux q, and IHTC. The analysis of the IHTC variation revealed a water content value within the range of 4 wt.% to 5 wt.% resulted in IHTC in two types of interpretation, called ‘fluctuation type’ and ‘turning type’.

Mass/heat transfer analogy method in the research of convective fluid flow through channels with a specific geometry

The present work contains the results of the comparative analysis of the literature data and the own investigations on mass and heat transfer coefficients occurring under conditions of the convective fluid flow through channels characterised by a specific geometry. The authors focused on the available experimental investigations on mass transfer. The considered experiments were carried out using the electrochemical method named limiting current technique. Two channel geometries were discussed: the annular channel of the conventional size and the long minichannel with a square cross-section area. Taking into consideration dimensionless numbers: Schmidt and Nusselt – analogical for mass and heat transfer – the formulas describing the phenomena under consideration were included. In the case of the annular channel the laminar and turbulent range of Reynolds numbers was studied. For the square minichannel – the laminar flow is considered. The analogy between mass and heat transfer introduced by Chilton and Colburn was applied in the analysis. An equivalent boundary condition is included in considerations concerning the mass and heat transfer. It is the Dirichlet boundary condition characterised by constant temperature of the wall which corresponds to the situation of constant ion concentration at the cathode surface in the limiting current technique. The main purpose of the present work was to verify the method for the determination of heat transfer coefficients using the analogy between mass and heat transfer processes in the case of convective fluid flow through the annular and square channels. The problem discussed in the present work is important and actual due to the possibility of the elimination of temperature measurements in the investigations of heat transfer processes occurring in channels characterised by a specific geometry. It should be noted that sometimes temperature measurement may be difficult or even impossible. This situation also causes high uncertainty of the obtained results. Due to this problem, the presented analysis was performed also with the use of thermal results based on the analytical solution. The verification of the use of mass/heat transfer analogy method in specific cases gives the extended knowledge of correct application of the limiting current technique in heat transfer research. The main objective of the work was achieved by conducting a comparative analysis of the adequate mass and heat transfer results. The existing literature data do not provide an answer to the question about the correctness of using the limiting current technique in the case of discussed annular channels or long square minichannels. The received results make us be critical of applying the mass/heat transfer analogy method in some heat transfer cases.

Precision injection moulding of micro components: Determination of heat transfer coefficient and precision process simulation

In polymer melt processing, the heat transfer coefficient (HTC) determines heat flux across the polymer/mould interface for both conventional and micro injection moulding (µIM), and is a key parameter in mould design and predictive computational simulations. However, values typically used in commercial software for injection moulding are usually obtained from large-scale experiments, which may be not appropriate for µIM. The present work introduces a novel experimental method for precisely determining the actual averaged HTC values in µIM. This method uses changeable mould inserts equipped with temperature and pressure sensors, positioned exactly at the same mould cavity location rather than at adjacent positions. The calculated HTC values obtained through this method are used subsequently in the simulation of µIM instead of software default values, following optimization of boundary conditions and mesh. This constitutes a significant addition to current knowledge in the field. The influence of injection velocity, packing pressure and mould temperature on HTC is studied. Experimentally calculated HTC values for the packing and cooling stages are approximately 8500 W/m2K and 6300 W/m2K, respectively. Based on process monitoring from short shot experiments, subsequent simulation of a 600 µm thick part is predicted successfully using multi-scale finite element modelling. Wall slip and venting parameters are optimized using a one-factor-at-a-time method; a friction coefficient of 0 and no venting are found to be the optimal parameters for modelling wall slip and venting. Both are critical in simulations, although wall slip has a greater influence than venting on the simulation accuracy. The default and calculated HTC values are used in the simulations for comparison, while the simulations using the calculated HTC values were validated as being effective and accurate.

Investigation and Application of Inverse Determination of Interfacial Heat Transfer Coefficient in Vacuum Investment Casting

Investigation and Application of Inverse Determination of Interfacial Heat Transfer Coefficient in Vacuum Investment Casting

ANALYSIS OF CALCULATION AND EXPERIMENTAL METHODS OF DETERMINING HEAT LOSSES THROUGH WINDOW CONSTRUCTION

Problem statement. Energy conservation in the conditions of deficit and constant growth in the cost of energy carriers is the main task of the housing and communal complex. Along with the use of energy-saving equipment in life support systems, a significant reduction in energy consumption can be achieved by introducing energy-efficient structural solutions into construction. Translucent structures are considered more unprotected, since heat losses through them can reach 54 % of the total, so they require new approaches to determining heat transfer coefficients. On the basis of the analysis of heat and mass transfer processes element by element for window blocks of non-standard shape, a correct estimate of the heat transfer coefficient was made. The purpose of the article on the basis of the analysis of the thermophysical processes formation through translucent enclosing structures, improve the dependences for calculating the heat transfer coefficient of double-glazed structures taking into acount the climatic load and establishing the relationship between their main thermotechnical characteristics. Results. Based on the analysis of thermophysical processes occurring in opaque structures, the dependence of the heat transfer coefficient of non-standard shape window block on the wind speed, taking into acount the orientation of the building height in the element-by-element determination, was obtained. The main factors affecting the value of the heat transfer coefficient of the window unit have been established, i. e., these dependencies can be used to evaluate energy-saving measures during an energy audit. Scientific novelty and practical value. Dependencies for determining the heat transfer coefficient of translucent structures, taking into acount heat loss through window frames, the location of the window in the wall opening, and structural solutions are obtained. An analysis of the results of heat transfer coefficients for windows of non-standard shape, which are aimed at choosing the optimal technical solutions of wine blocks from the standpoint of energy and resource saving, taking into acount climatic parameters, was conducted. Quantitative results of the radiant and convective components for the element-by-element determination of the heat transfer coefficient of window blocks and heat losses through them are obtained, allowing to establish the priority of energy-saving measures.

Modeling and simulation of the thermal behavior and electrical performance of PV modules under different environment and operating conditions

To adequately estimate the performance of a photovoltaic panel (PV) under actual field conditions, the present study tackles this problem by a multi-physics approach, where a thermal model based on a 3D finite-elements (FEM) method, and a combined optical−radiative one, are coupled to an electrical model, yielding the global simulation model. A detailed study regarding the determination of the convective heat transfer coefficients led to the proposal of a numerical algorithm to solve the non−linear equations obtained from the energy balances. The validation is done by comparing the present numerical results to a set of selected experimental data measured in the desert regions (Qatar and Algeria). As for the most relevant results, it is found that the nature of encapsulant material has almost no effect on PV performance, and for an efficient way of cooling the PV, a minimum distance of 150 mm between the ground level and the front face of the PV is recommended. The simulation results confirm that Adrar is one of the best sites in the world for solar applications, with the overheating of the PV cells largely compensated by the high potential of solar radiation.

Determination of Heat Transfer Coefficient in Casting: A Comparison of Analytical, Inverse and Trial-and-Error Methods

The Heat Transfer Coefficient (HTC) in casting is critical input in numerical simulations. However, it depends on alloy composition, melt temperature, mold preheating temperature and local casting modulus, hence it is difficult to determine a priori. This work uses temperature measurements obtained by thermocouples during investment casting of a multiple section reference part. HTC was determined by three different methods: (a) analytical, formulating a detailed expression for the HTC in time steps for a one-dimensional approximation (b) inverse, following a well-known iterative algorithm proposed in literature and (c) "trial-and-error" simulation runs for different HTCs based on the judgment of the analyst. In conclusion, the analytical method proved to be fast, but yields very approximate HTC. The inverse method achieves accurate temperature evolution, but it might be unrealistic in physics terms. The trial-and-error method is flexible and may be accurate yet cumbersome and uncertain, unless automated, for instance through a Genetic Algorithm.

Trefftz functions methods in analysis of the effect of enhanced heated surface on FC-72 flow boiling in minichannels

This paper focuses on time-dependent flow boiling heat transfer. The main element of the research stand was the test section with 11 rectangular minichannels of 1 mm depth. The FC-72 was heated by a thin foil during upward flow. The temperature of the outer surface of the foil was measured by infrared thermography. Two types of foil surface were tested: smooth and enhanced with recesses produced using emery paper. The main objective was to verify whether the enhanced surface helps to obtain a better heat transfer efficiency compared to the smooth surface. The 2D mathematical model of heat transfer was proposed, while methods with Trefftz functions were applied to solve the time-dependent problem of heat transfer. The Robin condition was used to determine heat transfer coefficients at the interface between the foil and the fluid. Experimental data on temperature and pressure and their validation were presented and discussed. Results based on the heat transfer coefficients were provided. They covered the calculation procedure, the determination of the mean relative error and results validation with using the 1D approach. The heat transfer coefficients as a consequence of application of the Trefftz method and the hybrid Picard-Trefftz method in the calculation procedure with comparison analysis of the results were shown. Furthermore, comparisons aimed at obtaining the results characteristic for smooth and enhanced surfaces were also conducted. In the experiment with an enhanced heated surface, the values of the heat transfer coefficient appeared to be higher. Boiling curves were illustrated and discussed.

Determination of diffusion coefficient and convective heat transfer coefficient for non-isothermal desorption-diffusion of gas from coal particles

Experiments investigating gas adsorption/desorption-diffusion in coal particles typically involve putting coal particles into a sample cell in a water bath to ensure a constant-temperature environment. Numerical simulations of such experiments have also been performed under isothermal condition. However, the coal particle temperature is not constant, and the influence of temperature variation on the diffusion coefficient back-calculation is often neglected. In this study, CO2 desorption-diffusion experiments are conducted with coal particles first, using the abovementioned water bath technique, and the coal particle temperature is monitored. Then, a coupled thermal-gas model for CO2 adsorption/desorption-diffusion in coal particle is proposed, in which the interaction between gas adsorption/desorption and coal temperature variation is considered. Finally, numerical simulations of the CO2 desorption-diffusion experiments are described, and the applied thermal boundary conditions are validated. The impacts of temperature variation on the desorbed-diffused gas amount and the diffusion coefficient back-calculation are also evaluated. The experimental results show that the coal particle temperature first decreases and then increases. Thus, setting an isothermal boundary condition for the particle surface will not lead to an accurate simulation of the temperature variation, which will ultimately also affect the accuracy of the diffusion coefficient back-calculation. Instead, adopting a convective heat boundary condition for the particle surface can better reproduce the evolution of desorbed-diffused gas and coal temperature changes, thereby improving the accuracy of the diffusion coefficient back-calculation. The results also indicate that the magnitude of coal diffusion coefficient and the size of coal particle both have significant effects on the temperature variation and the subsequent desorption-diffusion of adsorbed gas.

An efficient method for estimating time-varying convective heat transfer coefficient based on boundary condition transfer technique

Accurate determination of time-varying convective heat transfer coefficient (CHTC) is the main content of many engineering applications. In view of some problems existing in the determination of time-varying CHTC, this paper develops an estimation scheme using the boundary condition transfer technique. In the new scheme, the estimation of CHTC is transformed into the estimation of known fluid temperature. Then, according to the estimation results of fluid temperature, the discrete values of characteristic variable are weighted comprehensively to obtain the estimated results of time-varying CHTC. Numerical simulation tests are mainly adopted so as to validate the above scheme. The developed scheme is also compared with the traditional method, which can better reveal the performance of the former. Effects of some important parameters such as the number of unknown variables, fluid temperature and measurement noise on the estimation results are illustrated specifically. Results demonstrate that the proposal can effectively determine the time-varying CHTC and improve the computational efficiency. Moreover, the proposed method also reduces the effects of fluid temperature on the forecast results, which is helpful for the accurate determination of time-varying CHTC.

Thermal Effects in the End-Winding Region of Electrical Machines

The main heat transfer mechanism in the end-winding region of electrical machines is convection. In order to increase the air motion, the rotor is equipped with a series of blades. Their geometry is reflected in the fanning factor, i.e., the ratio between the rotor peripheral speed and air velocity. An accurate calculation procedure for the fanning factor has not yet been given. Knowing its value is crucial for the determination of air velocity and heat transfer coefficient (HTC), as the latter describes the end-winding heat removal capability. In this study, the convective heat transfer phenomena between the end winding and air inside the end-winding region were analyzed, with the heat generated only in the end winding, mimicked with a custom designed coil, and air moved by the blades. The analysis was performed by experimental testing and computational fluid dynamics (CFD) modeling. Measurements data were used to build a reliable CFD model. Further on, CFD results were used to derive a generalized analytical equation for calculation of the end-winding HTC, related to blade geometry and rotor rotational speed. The developed analytical model significantly improves the quality of real-time lumped circuit thermal modeling of electrical machines and, thus, enriches this field of science.

Evaluation of bed-to-tube heat transfer in a fluidized bed heat exchanger in a 75 MWth CFB boiler for municipal solid waste fuels

Bed-to-tube heat transfer has been investigated for a tertiary superheater in a 75 MWth Circulating Fluidized Bed (CFB) boiler in Norrköping, Sweden. The boiler is used for incineration of solid waste fuels. Two fluidized bed heat exchangers are located in loop seals, connecting the cyclones and the furnace. The heat exchangers are placed in series, with respect to the steam side, and in parallel, with respect to the particle side. The total heat transfer surface area is roughly 44 m2, distributed over 72 tubes. The total effect transferred most often is in the range 2–6 MW. The incoming steam temperature in the first superheater is 380–400 °C, while the exiting steam temperature from the second is around 450°, at 65 bar pressure. The bed temperature in the Fluidized Bed Heat Exchanger (FBHE) is 850–875 °C. The analysis is based on operational data from two time periods (2002–2005 and 2014–2021). The two periods use different heat exchanger designs, following a retrofit in 2005. The aim of the study is to establish the bed-to-tube heat transfer coefficient in an industrial FBHE unit and investigate how it varies over different time periods, for two different bed materials and for two different designs. Also, the experimentally determined heat transfer coefficients are compared with an established heat-transfer correlation, for prediction of heat transfer from bubbling fluidized bed to tubes. Operation with two bed materials were evaluated, namely silica sand and crushed and beneficiated ilmenite. Both materials are classified as Geldart B particles. Air is used as fluidization gas in the FBHE. The analysis show, with a few exceptions, comparably low heat-transfer coefficients from bed to tube of 100–150 W/(m2K). The results were similar for silica sand and ilmenite, but the highest measured heat transfer coefficient was for a period with ilmenite. The heat transfer was lower than expected based on literature data from FBHE units and fluidized bed boilers in general, and much lower than bed-to-tube heat transfer coefficients from lab-scale experiments and empirically derived predictive expressions. The difference could be related to one or more of several factors, such as the effect of very small tube spacing, unknown thermal conductivity of one of the layers in the tube bundle, the effect of lateral particle flow and the effect of fouling due to ash layers forming on the tube surfaces. It is suggested that it should be possible to significantly increase the bed-to-tube heat transfer by increasing the tube pitch, which is expected to improve bed mixing without increasing the risk of corrosion.

Experimental determination of the resistance to heat transfer of various building envelope constructions

The aim of the current research is to examine instruments for measuring heat flow and temperature, and to present methods for calculating resistance to heat transfer in accordance with regulatory documents. Requirements for constructions chosen as samples, and the conditions for their testing under stationary, quasi-stationary, and non-stationary conditions, are discussed. The study investigates devices for measuring thermal characteristics based on the research conducted by Yaping Cui, Jingchao Xie, Jiaping Liu, Peng Xue. An experiment performed by Chinese scientists to determine heat transfer coefficients using a naphthalene sample is analyzed. The research also presents a field experiment conducted by E.A. Gnezdilova to calculate the design characteristics of heat loss through floor constructions and building envelope constructions in direct contact with the soil. The validity and reliability of applying various methodologies are justified.

The local heat transfer coefficient determination during boiling a subcooled liquid on a superheated surface by the gradient heatmetry

The local heat transfer coefficient determination during boiling a subcooled liquid on a superheated surface by the gradient heatmetry

Determination of the casting-mold interface heat transfer coefficient for numerically die-casting process depending on different mold temperatures

Determination of the casting-mold interface heat transfer coefficient for numerically die-casting process depending on different mold temperatures

A pseudo-local heat transfer approach in a low tube to particle diameter ratio packed bed catalytic reactor: Oxidative dehydrogenation of ethane as a case study

A generalized approach to model heat transfer in an industrial-scale wall-cooled packed bed reactor with a low tube-to-particle diameter ratio (<8) is proposed and assessed in this work. The modeling approach rests on the realization of experiments carried out in absence of reaction in bench-scale as well as in industrial-scale packed beds. The approximation overcomes historical limitations identified when modeling radial heat transfer mechanisms by applying conventional approaches. The methodology leads to the reliable determination of the external wall heat transfer coefficient and pseudo-local radial effective thermal conductivity. The approach allows the quantification of heat transfer resistances through the core, the internal and the external wall of the bed indicating that approximately 30 % of the resistances are located along the internal side of the packed bed when it was operated at Particle Reynolds numbers ranging from 700 to 1400. Because of its complex impact on heat transfer, fluid dynamics is accounted for by implementing a methodology that uses pressure drop data and the mass conservation criterion to describe velocity profiles, including the determination of the viscous and inertial resistances caused by solid surfaces at the core and near the wall. Finally, the heat transfer information is transferred to a pseudo-heterogenous model to simulate the performance of an industrial-scale wall-cooled packed bed reactor for the highly exothermic oxidative dehydrogenation of ethane. Simulations demonstrate both the reliability of the proposed heat transfer approach and the limitations of the conventional approximations when describing temperature profiles in a packed bed reactor.

Determination of Secondary Cooling Zone Heat Transfer Coefficient with Different Alloy Types and Roughness in DC Casting by Inverse Heat Conduction Method

The cooling characteristic curves in a heated ingot with a diameter of 100 mm quenched by a water jet were measured under different conditions. A two-dimensional calculation model was established to calculate the HTC of magnesium alloy ingot with water spray cooling. Data from cast-in thermocouples trail were input into the model and the HTCs were back-calculated for the water quench region. The HTCs were calculated under different alloy types and roughness conditions, and the relationship between the ingot surface temperature, roughness, and the HTC was established accordingly. The results show that the greater the thermal conductivity of the alloy, the greater the heat transfer coefficient (HTC). The HTCs of AZ80, AZ31, and ZK60 alloys increase successively. With the decrease in the surface temperature, the HTC in both the impingement zone and the free−falling zone shows a trend of a rapid increase at first, then slowly increasing to the maximum value, and finally, a rapid decrease, with the peak value appearing at about 400 K. Considering the influence of the ingot surface temperature and the surface roughness on the HTC, the mathematical relationships between them are established.

Mathematical Model of Air Dryer Heat Pump Exchangers

This paper presents a mathematical model of heat pump exchangers and their thermal interaction with a fan for an air dryer. The calculation algorithm developed for the finned heat exchangers is based on the ε-NTU method, allowing the determination of air side and refrigerant side heat transfer coefficients, evaporator and condenser heat capacity and air parameters at the dehumidifier outlet with known exchanger geometries, initial air parameters and mass flow rate. The model was verified on an industrial dehumidifier test bench. This enabled the heat transfer coefficients for the exchanger to be calculated as a function of the speed and, therefore, the power of the fan’s drive motor. An increase in fan performance on the one hand results in an increase in the heat transfer rate, but, on the other hand, it causes an increase in the total energy consumption of the motor. Thus, while it causes an increase in drying capacity, it also causes an increase in the energy consumption of the dehumidifier. In order to optimise the unit in terms of energy consumption, it is therefore necessary to determine a function that relates the amount of heat exchanged to the efficiency of the fan.