External Convective Coefficient Research Articles

Numerical investigations on the absorption of a metal hydride hydrogen storage tank based on various thermal management strategies

This study presents numerical investigations on the performance of the charging process of a metal hydride-based hydrogen storage tank. Three scenarios were developed to examine the absorption performance and compare the effectivenesses of proposed thermal management alternatives. In Scenario-1, the metal hydride (MH) tank is cooled with convective heat transfer on the lateral surface of the tank, i.e., external convection. In Scenario-2, phase change material (PCM) is wrapped around the MH tank to investigate passive thermal management. Scenario-3, a hybrid approach, includes the PCM layer and an internal cooling pipe. In this scenario, the heat transfer fluid (HTF) inside the tube is activated depending on the temperature within the MH tank. An in-house 2D axisymmetric numerical code was developed using the finite volume approach in MATLAB software. In the parametric study, several design parameters and operating conditions were selected, such as (i) charging pressure, (ii) external convective heat transfer coefficient, (iii) PCM thickness, (iv) latent heat of PCM, (v) HTF temperature, and (vi) convective heat transfer coefficient of the HTF, and as a total of 334 parametric analyses were conducted by varying the numerical values of the parameters chosen in wide ranges. The outputs, like spatial and temporal variations of hydride bed temperature and weight fraction, are evaluated for each of the proposed scenarios. As a result, it was observed that the storage performance strongly depends on the convection coefficient at ambient conditions. In Scenario-1, beyond 1000 W/m2K, convective heat transfer coefficient variations do not cause a significant change in the charging process. In Scenario-2, variations in PCM thickness do not cause a considerable change in the storage performance significantly beyond 3 cm. The variations in PCM thickness become important at lower latent heat values and PCM thicknesses less than 2.5 cm. In Scenario-2 and Scenario-3, PCM thickness over 2.5 cm and latent heat of fusion parameter over 150 kJ/kg should be selected to provide a thermal management strategy to be implemented. While the storage criteria were determined according to different thermal management strategies, the PCM thickness and latent heat of fusion parameters showed higher storage performance than the external convection coefficient at the same charging pressure. Temperatures at critical nodes in the MH-PCM system vary depending on location and reaction time. For all thermal management strategies in the hydride bed in positional temperature distributions, the temperature is minimum at the nodes close to the straight pipe. In the scenario-based analysis of the hydride bed temperature, the Scenario-3 thermal management strategy provides the MH bed domain at a lower temperature than Scenario-2. Here, the convection coefficient inside the cooling channel is a critical parameter for the hydride bed temperature.

Identifying key parameters through a sensitivity analysis for realistic hygrothermal simulations at wall level supported by monitored data

The reliability of hygrothermal simulations of building components is key for designing energy efficiency measures, assessing living comfort, and preventing building damage. The model accuracy is related to the reliability of the selection of input parameters. Due to the high uncertainty, the selection of the input values is challenging. This work aims to calibrate a hygrothermal simulation model exploiting monitored values recorded in a case study located in Settequerce (Italy), to understand how close to reality a numerical model can be. Moreover, a sensitivity analysis, based on the Morris method together with a Latin Hypercube sampling, is applied to identify the input parameters that affect most significantly the simulation. The results of the calibration indicated that is possible to obtain reliable outputs by appropriately selecting materials within the database. The sensitivity analysis showed that the relative humidity under the insulation is largely influenced by the water vapor diffusion resistance factor of the plaster, applied during the renovation phase both on the internal and external side. Among the coefficients describing the coupling with the boundary conditions, only the external convective heat coefficient and the coefficient of short-wave solar radiation influence slightly the objective function.

Transient heat pipe failure accident analysis of a megawatt heat pipe cooled reactor

Heat pipe cooled reactors, which use heat pipes rather than fluid flow for passive cooling, are an excellent candidate for micro nuclear power sources. Heat pipes provide excellent built-in redundancy and safety margin. However, a heat pipe failure is quite likely over the reactor lifetime, which is one of the essential design basis accidents in the reactor design. This work analyzes heat pipe failure accidents to investigate the system transient performance and the response of a heat pipe cooled reactor. Single heat pipe failure and cascading heat pipe failure are simulated and analyzed by establishing a transient analysis code, including a point reactor kinetics model, a core thermal-mechanical model, and a heat pipe model. The simulation shows that in the single heat pipe failure accident, the peak monolith temperature at the failed channel increases and is only 8 K lower than the fuel center temperature. In the cascading heat pipe failure accident, the temperatures in each channel suddenly increase over a short period and rise to the maximum at 22,900 s as the heat losses increase and the core power decreases. The sensitivity of the external convection for residual heat removal is also analyzed. For small external convection coefficients less than 100 W/m2K, increasing the external convection coefficient significantly reduces the maximum temperatures. By contrast, for external convection coefficient greater than 500 W/m2K, increasing external convection coefficient still reduces the core peak temperature, but the effect is weak.

Simulating onset and evolution of thermal runaway in Li-ion cells using a coupled thermal and venting model

A coupled, thermal and gas generation/venting model has been developed for simulating the onset and evolution of thermal runaway in 18650 format Li-ion battery cells. The model simulates heat and gas generation during external heating of an electrically isolated cell that results in thermal runaway. Gas generation within the cell leads to pressure build up until the point at which the vent mechanism opens and relieves the internal pressure. Compressible flow of gases is modeled through the vent cap as a function of pressure ratio across the vent. The energy balance of the battery cell includes: heat generated from decomposition reactions and electrical short, external heat transfer to the surroundings, heat absorbed with vaporization and melting processes, as well as the energy loss as material is vented from the cell. The model was able to capture features of the temperature evolution of the battery cell well and generate detailed information about the progression of thermal runaway. The model was exercised to simulate time-to-venting and time-to-thermal-runaway for various changes in cell design parameters such as: amount of free liquid electrolyte, external convection coefficient, and electrolyte evaporation rate after vent opening.

Numerical modeling of the thermal conduction process in water-air convector’s fins

The paper presents results from numerical modelling of thermal conduction process in ABAQUS. The computing mesh of the model has been verified. Based on the simulation results, the average temperature of the heat exchanger’s external surface and the external heat convection coefficient have been determined. The process of thermal conduction in the fins is modelled on the basis of boundary conditions, which values are obtained by simulation of the heat convection process in the convector’s pipes. The average heat parameters have been determined relative to the first, middle and last fins, by developing a geometric model in ABAQUS. The computing mesh has been verified by increasing the number of cells and nodes. The external heat convection coefficient is calculated by Newton's law.

Finned Electric Motor with Prescribed Heat Flux and Influence of the Internal and External Heat Convection Coefficients on the Temperature of the Core

One of a major objective is to analyze the effect of internal and external convection coefficients in the heat transfer generated by electric motors and characterize intervals of feasible values, in practical terms, for these coefficients. Another objective is to determine the motor core temperature, considering the environmental and operational conditions of the motor installation and the heat flux Q0 (fixed) in the fins. To achieve the objectives were developed analytical solutions for determining the temperature variations in the fins, performance and electric motor efficiency, considering heat flow constant at the base of the fins and the possible variations in temperature of surround media. The heat flux at the base of the fins, in this case, is the minimum necessary for satisfactory electric motor performance and the core temperature is within the safety range stipulated by the manufacturer. The obtained results characterize a range of possible values for the inner and outer heat transfer coefficients.

Heat transfer coefficients of double skin facade windows

This article studies the heat transfer coefficient of double skin facade (DSF). Experiments were conducted for a lab-scale DSF model of 1.1 m high. The measured data were used to validate a computational fluid dynamics (CFD) model, which was then used to study the full-scale DSF. The experimental data showed that closing the cavity reduced the effective heat transfer coefficient by about 25%. In the ventilated mode, the DSF could have better insulation than double glazing (DG) and an optimal cavity depth existed for the lab-scale DSF. There were cases where the DSF had a higher heat transfer coefficient than DG. In these cases, the induced natural ventilation in the cavity may have caused a greater convective coefficient than that at the external surface of the facade. For the U value, which is evaluated at a high external heat transfer coefficient, the simulation results show that the DSF is always better than DG and the influence of the size of the cavity depth does not appear important. In normal applications of facade, the typical external convective heat coefficient is usually large due to the ambient wind. In this case, the DSF will have better insulation than the single facade of DG.

Numerical Feasibility Study of a Wax Cold Flow Approach for Subsea Tie-In Flowlines Using a 1D Mechanistic Multiphase Flow Simulator

This work is a thermo-fluid numerical case study to investigate the size and performance of a system that eliminates needs for insulating, heating and inhibiting chemically the deposition of wax in subsea tie-in flowlines. For short, we call this type of systems “Cold Flow”. The particular system analyzed in this study consists of a reactor unit at the inlet to the flowline, where the thermal solubility of the wax-creating molecules is reduced by cooling. Subsequently, solid wax is deposited in the reactor piping and wax free crude is entering the flowline. The reactor is regenerated periodically. The reactor-pipeline system was modelled using a commercial flowline simulator, with transient, thermal, multiphase and deposition capabilities. The basic layout used was a transportation pipeline of 8 km and 6.69 in ID with a mass flow rate of 17.51 kg/s, a water cut (WC) of zero and an inlet temperature of 70°C. The wax appearance temperature (cloud point) of the crude is 22°C and the seabed temperature is 4°C. Three types of reactors have been simulated: a non-insulated pipe section, a passive cooler with a bundle of parallel pipes and an active cooler. Sensitivity analyses have been performed for all three cases varying the external convective coefficient, the reactor pipeline diameter and the WC. For a non-insulated pipeline section cooler, the required length is of the same order of magnitude as the main flowline, implying that such a solution is impractical for short flowline distances or when a compact deployment is desired. For the passive cooler case, the required length was half of that in the previous case; thus it is still significant. For the active cooler reactor, the required cooling duty was 2.2 MW. In all three cases, the pipe-flow dynamics were analyzed, and the pigging arrangement complexity has been qualitatively addressed. However, the detailed design falls out of the scope of this study.

Heat transfer study of external convective and radiative coefficients for building applications

In the building sector, the assessment of heat transfer processes is essential to identify the performance of structural elements. Heat transfer occurring in internal and external environments between walls and surrounding bodies is quantified by means of convective and radiative coefficients able to summarize physical phenomena. In building applications, International Standards suggest to calculate thermal transmittances by proposing specific surface thermal resistance values.Moreover, several equations have been developed in scientific literature to compute the convective and radiative heat transfer coefficients, particularly useful in the building simulation field. For what concerns external coefficients, these correlations are generally related to the wind velocity, wind direction (windward and leeward surfaces) and surface roughness.In this paper, the actual total external heat transfer coefficients in different case studies were obtained by measuring the physical parameters needed to define them. Peculiar geometries, i.e. balconies and porticos, were addressed, for which specific correlations are not available in literature. Convective and radiative components were distinctly assessed and, finally, actual convective heat transfer coefficients were compared with the same coefficients obtained by applying the conventional correlations available in literature and with the constant value suggested by the Standard. In addition, the convective heat transfer coefficients were evaluated by means of empirically determined correlations based on dimensionless parameters. The final goal of this work is to investigate the dissimilarities among the different coefficients values, in order to analyze applicability and limits of UNI EN ISO 6946 and of existing correlations for external heat transfer coefficients.The analysis of the coefficients shows that the most significant differences are related to the convective heat transfer phenomena quantification. Applying the same correlations to different case studies scattered results were obtained whereas employing the dimensionless numbers correlations small oscillations in both case studies can be observed.

Heat transfer in a porous saturated wavy channel with asymmetric convective boundary conditions

The viscous flow in a wavy channel with convective boundary conditions is investigated. The channel is filled with a porous viscous fluid. Two cases of equal and different external convection coefficients on the walls are taken into account. Effect of viscous dissipation is also considered. The governing equations are derived employing long wavelength and low Reynolds number approximations. Exact closed form solutions are obtained for the simplified equations. Important physical features for peristaltic flow caused by the wavy wave are pumping, trapping and heat transfer rate at the channel walls. These are discussed one by one in depth and detail through graphical illustrations. Special attention has been given to the effects of convective boundary conditions. The results show that for Bi1≠Bi2, there exists a critical value of Brinkman number Brc at which the temperatures of both the walls become equal. And, for Bi1>Bi2 and Br>Brc, the temperature of the cold wall exceeds the temperature of hot wall.

A Transient Immersed Coil Heat Exchanger Model

The aim of this paper is to present a transient one-dimensional (1D) radial immersed coil heat exchanger model that accounts for the effect that geometry and operating conditions have on heat transfer performance. Insights gained through its use in both an analysis of experimental data and an implementation in the simulation environment TRNSYS are shown and discussed. While variation in the external convection coefficient of immersed coil heat exchangers has little effect on the annual solar fraction of a generic solar domestic hot water system, variation in collector side flow can influence the solar fraction as great as ±5%, in particular low collector side flow improves stratification inside the store.

Numerical study of natural convection in a liquefied natural gas tank

Natural convection in a liquefied natural gas storage tank, which is caused by the heat absorption from the surrounding environment, is studied numerically by solving the conservation equations for mass, momentum, and energy with a commercial computational fluid dynamics code, FLUENT. The present numerical results show that the interfacial heat transfer rate, which is directly related to the boil-offgas generation rate, strongly depends on the liquid-solid contact area. The contribution of the heat transfer rate from the vapor region is negligible compared with that from the liquid region. The effects of the external convection coefficient, tank size, and tank shape on the flow and temperature fields and on the interfacial heat flux are quantified.

Evaluation of external heat loss from a small-scale expander used in organic Rankine cycle

With the scaling down of the Organic Rankine Cycle (ORC), the engine shaft power is not only determined by the enthalpy drop in the expansion process but also the external heat loss from the expander. Theoretical and experimental support in evaluating small-scale expander heat loss is rare. This paper presents a quantitative study on the convection, radiation, and conduction heat transfer from a kW-scale expander. A mathematical model is built and validated. The results show that the external radiative or convective heat loss coefficient was about 3.2 or 7.0 W/K·m2 when the ORC operated around 100 °C. Radiative and convective heat loss coefficients increased as the expander operation temperature increased. Conductive heat loss due to the connection between the expander and the support accounted for a large proportion of the total heat loss. The fitting relationships between heat loss and mean temperature difference were established. It is suggested that low conductivity material be embodied in the support of expander. Mattress insulation for compact expander could be eliminated when the operation temperature is around 100 °C.

Reliable one-dimensional approximate solutions for insulated oblate spheroid containers

Since there is no reliable surface-area formula available for the calculation of the heat-transfer characteristics of an insulated oblate spheroid, in this investigation, highly accurate oblate spheroid surface area is obtained by a simple numerical integration method. Based on the accurate oblate spheroid surface area, a reliable one-dimensional approximate solution of an insulated oblate spheroid container can be obtained with a one-dimensional RPSWT model. By comparing with accurate three-dimensional numerical results, it is found that in cases of low external convective coefficient ( h o = 8.3 W m −2 K −1), long–short-axes ratio a / b ≦ 3 , and insulation thickness t / R 2 ≦ 2 , errors are within 3%. In rarer situations with a / b ≦ 5 and t / R 2 ≦ 2 , the errors are within 5.5%. The above results are almost independent with the internal flow convective coefficient ( h i from 30 to 10 5 W m −2 K −1) and dimensionless container size (0.5 R 2 h o/Ks from 1.55 to 155).

Fabrication of polymer film heat transfer elements for energy efficient multi-effect distillation

Conventional multi-effect and multi-stage flash distillation use aluminium brass, cupro-nickel and/or titanium heat transfer surfaces. Polyolefins such as high density polyethylene (HDPE) and polypropylene (PP) have better corrosion resistance than these, which permits thinner walls. For identical internal & external convection coefficients, 20–50 μm thick HDPE and PP film heat transfer elements have from 71–105% of the U value of 1 mm cupro-nickel tubes. Experience has shown them to last as well as — and in some high-scaling water re-use applications better than — titanium elements. But per unit area they cost far less. This low cost — especially when combined with durable low cost vacuum vessels — permits the installation of more thermal conductance (UA) than with metal heat transfer surfaces. This leads to a lower temperature difference Δ T 1 between the condensing and evaporating sides, and a lower specific energy consumption. We describe the impulse welding of thin HDPE and PP films on specially developed apparatuses to produce tube-like heat transfer elements. Some of these were pressure tested up to several bars at room temperature to determine the strength of the weld lines. We make brief remarks about the design, construction and testing of a simple falling saline film vapour compression desalinator — with vertically oriented air mattress-like multi-tube polyolefin film heat transfer elements. Designed for operation with a small Δ T 1. From the frictional pressure drop Δ p f of the condensing vapour for laminar flow inside a film tube, we relate the film tube diameter, length, U value, temperature and the ratio R T = Δ T 1/Δ T f of temperature difference Δ T 1 to frictional temperature drop Δ T f.

Investigations of the Thermal Spreading Effects of Rectangular Conduction Plates and Vapor Chamber

This study examines the spreading ability of rectangular plates numerically, analytically, and experimentally. The effect of aspect ratio, defined as an equivalent radius of a heater divided by that of a spreader plate, is investigated. The numerical results show a very good agreement with the analytical solutions. From the calculated results, the spreading resistance of the conduction plates with a small aspect ratio is higher than the one-dimensional conduction resistance. Calculated results also show that the spreading ability of a metal plate would be affected slightly by the external convective heat-transfer coefficient when the ratio of the longitudinal heat convection to the lateral heat spreading is less than 0.1. In addition to the numerical analysis, experimental comparisons between copper∕aluminum plates and a vapor chamber having the same thickness have been conducted. The experimental results show that the thermal resistance of the metal plates is independent of input power whereas that of the vapor chamber shows a noticeable drop with increased power. For the influence of concentrated heat source, the surface temperature distributions for the metal plates become concentrated with a reduced aspect ratio. However, the variations of the aspect ratio and the input power would yield minor effects to the surface temperature distribution of the vapor chamber. As compared with the conduction plates, the vapor chamber would offer a lower temperature rise and a more uniform temperature distribution. Thus, the vapor chamber provides a better choice as a heat spreader for concentrated heat sources.

Modeling Flow and Heat Transfer During Freezing of Foods in Forced Airstreams

ABSTRACT: Mathematical modeling of food freezing has been limited to the modeling of the internal heat transfer where the external convective heat‐transfer coefficients are assumed or empirically estimated. Previous procedures followed to solve the external boundary layer in tandem with the internal heat transfer were constrained by numerical complexities due to the transient nature of the heat transfer, requiring unsteady formulation for the flow. In this article, attempts have been made to decouple the flow and heat transfer equations for the external boundary layer flow over a food product being frozen. The flow equations have been solved as a steady‐state problem using Falker‐Skan transformations of the boundary layer equation. The heat‐transfer equation for fluid flow is solved as an unsteady‐state problem in conjunction with the internal heat transfer and phase change inside the product undergoing freezing. The model is validated for a case of air‐impingement freezing.

Heat removal of in‐tube viscous flows to air with the assistance of arrays of plate fins Part I: theoretical aspects involving 3‐D, 2‐D and 1‐D models

A detailed comparative study of the heat transfer augmentation of in‐tube flows accounting for an array of equally‐spaced plate fins attached at the outer surface is undertaken. The aim of the paper is to critically examine the thermal response of this kind of finned tubes to three different mathematical models: a complete 3‐D distributed model, a reduced 2‐D distributed/lumped hybrid model and two largely simplified 1‐D lumped models. For the three models tested, the computed results consistently demonstrate that the simplest 1‐D lumped model, with embedded arithmetic spatial‐ and geometric spatial‐means of the angular external convective coefficients provide dependable algebraic estimates of the actual heat transfer provided by the 3‐D distributed model with its indispensable finite‐difference solution. Further, an arithmetic mean of the maximum and minimum heat transfer supplied by the 1‐D lumped model delivered results that match those computed with the 3‐D distributed model. The most important steps of the mathematical derivations have been highlighted. A representative group of thermal performance diagrams is explained with the intent to assist engineers engaged in the thermal design of externally finned tubes of compact heat exchangers and HVAC devices.

Outside surface resistance: Proposed new value for building design

This paper presents a critique of the design guidance currently offered by the Chartered Institution of Building Services Engineers (CIBSE) and by the American Society of Heating, Refrigeration and Air-conditioning Engineers (ASHRAE) with respect to external convection heat transfer at building façades. Their guidance to date is based on wind tunnel studies. Values are deduced for the convection heat transfer coefficients that form the basis of current advice. Deductions are compared with the findings of outdoor full-scale experimental studies. It is shown that these experimentally measured values for the convection coefficient are significantly higher than the values assumed in current design guidance. Based on this evidence, an expression derived from four full-scale investigations is presented for use by designers and building thermal modellers for correlating the external convection coefficient with surface windspeed. Using the expression, a modified design value of 0.04m2K W -1 for the external surface resistance Rso is proposed for the condition of 'normal' exposure (based on a design surface windspeed of 2 m s-1 as defined by CIBSE) to replace the current value of 0.06 m2 K W-1. As a result, it is recommended that the corresponding U-value for single glazing be raised to 6.3 W m -2K-1 from its currently accepted value of 5.6W m-2 K-1. A summary of results is also presented, showing the effect of window framework on external convection heat transfer.

96/06503 External convection coefficients for framed rectangular elements on building facades

Electrical properties of refrigerants are of importance as soon as bushings are surrounded by a refrigerant. This is the case e. g. for hermetic sealed motor compressors as well as for some control devices such as liquid level control units or capacity controls for compressors. This paper presents a survey of existing data for HFC refrigerants and presents new measurements for those HFC blends that have been identified as long term replacements for CFCs and HCFCs. The data collection includes permittivity, electrical conductivity and breakdown voltage. Values are given for the HFC blends R404A, R407C, R410A and R507 as well as for R134a.Il est important de prendre en compte les propriétés électriques des frigorigènes lorsque les composants métalliques sont entourés d'un frigorigène. On rencontre cette situation dans le cas des compresseurs hermétiques et des appareils de commande tels que les appareils utilisés pour réguler le niveau du liquide ou, dans le cas des compresseurs, les systèmes de réglage de la puissance. Cette communication présente une enquête des données sur les frigorigènes HFC ainsi que de nouvelles mesures effectuées sur les mélanges de HFC destinés à remplacer les CFC et les HCFC dans le long terme. Les données présentées comprennent : la constante diélectrique, la conductivité électrique et la tension de claquage.