Wall Thermal Resistance Research Articles

Nucleate boiling in two types of vertical narrow channels

To explore the mechanism of boiling bubble dynamics in narrow channels, we investigate 2-mm wide Iand Z-shaped channels. The influence of wall contact angle on bubble generation and growth is studied using numerical simulation. The relationships between different channel shapes and the pressure drop are also examined, taking into account the effects of gravity, surface tension, and wall adhesion. The wall contact angle imposes considerable influence over the morphology of bubbles. The smaller the wall contact angle, the rounder the bubbles, and the less time the bubbles take to depart from the wall. Otherwise, the bubbles experience more difficulty in departure. Variations in the contact angle also affect the heat transfer coefficient. The greater the wall contact angle, the larger the bubble-covered area. Therefore, wall thermal resistance increases, bubble nucleation is suppressed, and the heat transfer coefficient is lowered. The role of surface tension in boiling heat transfer is considerably more important than that of gravity in narrow channels. The generation of bubbles dramatically disturbs the boundary layer, and the bubble bottom micro-layer can enhance heat transfer. The heat transfer coefficient of Zshaped channels is larger than that of the I-shaped type, and the pressure drop of the former is clearly higher.

Heat transfer in rectangular microchannels heat sink using nanofluids

The effect of using nanofluids on heat transfer and fluid flow characteristics in rectangular shaped microchannel heat sink (MCHS) is numerically investigated for Reynolds number range of 100–1000. In this study, the MCHS performance using alumina–water (Al 2O 3-H 2O) nanofluid with volume fraction ranged from 1% to 5% was used as a coolant is examined. The three-dimensional steady, laminar flow and heat transfer governing equations are solved using finite volume method. The MCHS performance is evaluated in terms of temperature profile, heat transfer coefficient, pressure drop, friction factor, wall shear stress and thermal resistance. The results reveal that when the volume fraction of nanoparticles is increased under the extreme heat flux, both the heat transfer coefficient and wall shear stress are increased while the thermal resistance of the MCHS is decreased. However, nanofluid with volume fraction of 5% could not be able to enhance the heat transfer or performing almost the same result as pure water. Therefore, the presence of nanoparticles could enhance the cooling of MCHS under the extreme heat flux conditions with the optimum value of nanoparticles. Only a slight increase in the pressure drop across the MCHS is found compared with the pure water-cooled MCHS.

Study for Bubble Dynamics of Nucleate Boiling in Narrow Channels

Two different types of channels are investigated which have I- and Z-shaped cross-sections with a width of 2mm. Using the numerical simulation method, the influence of wall contact angle to the process of bubble generating and growth up is studied, and the relationship between different channel shapes and pressure drop is also investigated. In the calculation process, the effects of gravity, surface tension and wall adhesion are taken into account. It is found that wall contact angle has a great influence to the morphology of bubbles. The smaller the wall contact angle is, the rounder the bubbles are, and the shorter the bubbles take to departure from the wall, otherwise, the bubbles are more difficult to depart. The variation of contact angle also has effect upon the heat transfer coefficient, the greater the wall contact angle is, the larger bubble-covered area is, thus the wall thermal resistance gets higher, and the heat transfer coefficient becomes lower. The role of surface tension in the process of boiling heat transfer is much larger than the gravity in narrow channels. The generation of bubbles dramatically disturbs the boundary layer, and the bubble bottom micro-layer can enhance the heat transfer. The heat transfer coefficient of Z-shaped channels is larger than that of I-shaped channels, while the pressure drop of the former is obviously higher.

A data analysis method for detecting wall thermal resistance considering wind velocity in situ

To improve the accuracy of data analysis methods for the field measurement of wall thermal resistance, a method considering wind velocity is presented through an analysis of both surface heat flux and temperature samples. This analysis method takes the wall heat transfer of linear system into account, simplifying the first order differential equation of wall transient response based on the nodes of the interior and exterior wall surfaces. An approximate solution has been proposed in the form of time domain interpolation. By applying the weighted residual method for the approximate solution, the data analysis method about temperature, heat flux, and wind velocity can be achieved. A transient heat transfer model of a wall was established applying the infinite difference method, by which the proposed analysis method was validated in this paper. Additionally, the heat flow meter experiment platform considering wind velocity was built, on which the proposed method, the mean method and the dynamic analysis method suggested by the international standard ISO 9869 were applied to the test wall under different wind velocities. The wall thermal resistance value obtained in our proposed method has proven to be in better agreement with that obtained for a steady state.

Numerical study of the transient behaviour of a thermal storage module containing phase-change material

In this paper, the transient behaviour of a thermal storage module is studied numerically based on the enthalpy transforming method. The module is composed of a concentric tube, in which the annulus contains the phase-change material (PCM) and the inner tube carries the heat transfer fluid. The full set of governing equations accounting for heat and fluid flow in the inner tube, the change of phase of the PCM in the annulus, and heat conduction in the walls are solved simultaneously as a conjugate problem. A parametric study, based on three different PCMs, one with hypothetical constant properties and the other two with variable properties, is conducted to investigate the effects of outer wall thickness and variable thermo-physical properties of the PCM on the performance of the storage module during the phase-change process. It is observed that property variation effects become more pronounced as the Stefan number increases. Interestingly, more latent heat can be stored at a given time as the thermal diffusivity ratio is increased for both constant and variable property cases. This can prevent the over-sizing of the system based on constant property data. Another feature of considerable interests is the delay in the charging time: the time required for the PCM to store a certain amount of energy when the wall thermal resistance is included. A sample set of predicted data shows an error of at least 22 and/or 7.7 per cent when the property variation and/or wall thermal resistance is neglected, respectively.

A numerical model of thermal analysis for woven wire screen matrix heat exchanger

Woven wire screen matrix heat exchanger (WSMHE) is a kind of compact, light-weight and high-efficiency matrix heat exchanger (MHE) for cryogenic applications. This paper presented a numerical model for the design and thermal analysis of WSMHE. The influence of wall thermal resistance, axial conduction, parasitic heat load and properties variation was taken into account, which is neglected in the traditional effectiveness– NTU method but important for compact cryogenic heat exchangers. The proposed numerical method is verified by effectiveness– NTU method under specific conditions, then it is tested according to experiment data, and a well agreement was obtained. Based on this model, a detailed analysis was performed on WSMHEs. The analysis results show that the axial conduction might result in evident decrease of effectiveness at low flow rate region; the effectiveness of WSMHEs with large flow rate could be remarkably enhanced by increasing its length; the influence of parasitic heat load varied little throughout of the flow rate region. Furthermore, the numerical model presented in this paper can be developed to the design and thermal analysis of small partition wall type heat exchangers.

The effect of reinforced concrete frames on the thermal performance of residential villas in hot climates

This paper investigates the impact of reinforced concrete (RC) frames and structures on the thermal performance of a typical residential villa in Kuwait. In the approach, a simplified method to calculate the effective overall heat transfer coefficient and the overall thermal resistance of a construction was used in conjunction with a comprehensive building energy simulation program (ESP-r), to predict the impact of RC on the thermal performance of the villa when subjected to different constraints. It was found that for the worst case, the calculated overall thermal resistance of walls incorporating RC was lower by 64% than the minimum value stipulated in the code. On the other hand, the reductions attained in the air-conditioning peak power requirement, due to improved application of thermal insulation on RC structures, ranged from 3% to 12%. The cost benefit analysis indicated that thermal insulation of RC structures of residential buildings, lead to substantial savings for the government, mainly due to the high subsidy on the cost of electricity.

Steady state theoretical model of fired clay hollow bricks for enhanced external wall thermal insulation

This paper proposes a theoretical model to study the steady state thermal behavior of fired clay hollow bricks for enhanced external wall thermal insulation. The study aims at the development of new materials and structural components with good thermal material properties, with respect to energy saving and ecological design. Thermal insulation capacity of two external walls of different thicknesses, constructed of locally produced bricks, is studied. The basic brick units used for the investigation are small-size bricks with eight equal cavities or recesses and big-size bricks with twelve equal recesses. Their recesses configuration has been varied to perform the assessment. The insulation materials injected within brick recesses during the assessment are granulated cork and expanded polystyrene. The improvement in the thermal performance of the walls will be the result of optimization among the various factors such as brick cavity configurations, integration of insulation within brick recesses and the cavity surface emissivities. So emphasis is given to the study of the impact of these factors singly or in combination on the overall thermal resistance of walls in order to find out the best design solutions to maximize their thermal insulation capacity. Computer modeling and calculations performed, for steady state conditions, show that the increase in hollow brick cavity height contributes to the improvement of the overall thermal resistance of the order of 18–20%. The improvement could significantly increase to the range of 88.64% and 93.33%, if the bricks used are injected with the insulating material. If the cavity surface emissivities are lowered to 0.3, the improvement will be 72.73–78.33%. The results have also shown that replacing the cork by expanded polystyrene (EPS), having lower thermal conductivity, would not improve significantly the overall thermal resistance. This improvement is 9.08% for a wall of small-size bricks having configuration BS2CV and 8.34% for a wall of big-size bricks having configuration BB3CV.

A study of capillary-assisted evaporators

The paper presents a comparative study of the effect of porous layer parameters on the thermal performance of capillary-assisted evaporators. The results show that there are optimum particle sizes that are associated with maximum boiling heat rates and heat transfer coefficients. The results also show that the layer thickness has monotonic effect on the evaporators’ performance but its impact is less pronounced compared to that of the particle size. The trends are explained in terms of the contradicting effects that exist within the layer. Agreement in trend, and to a lesser degree in value, is obtained between the analytical and experimental results with the discrepancy explained in terms of the inaccuracies of the empirical formulae used and the effect of condensation on the tube wall thermal resistance. Boiling heat transfer coefficients of up to 5 kW/m 2 °C were achieved.

Parametric space distribution effects of wall heat capacity and thermal resistance on the dynamic thermal behavior of walls and structures

The present work aims at the investigation of the combined space distribution effects of heat capacity and thermal resistance on the transient thermal behavior of a wall, seen as a continuum of distributed parameters. The physical system is compared with the idealized wall lumped parameter model and its thermal time constant is related to its effective wall heat capacity, defined as the fraction of the wall heat capacity which participates in a transient thermal process. The effect of the space distribution of heat capacity and thermal resistance on the effective wall heat capacity is investigated for a wide range of homogeneous and multilayer thermally insulated walls. It is derived that the decrease of thermal resistance in homogeneous walls leads to an increase of their effective heat capacity. However, the effects are remarkably stronger on the effective heat capacity of thermally insulated multilayer walls, in which when the thermal insulation layer is at the ambient side, it leads to a significant increase of effective heat capacity, although when it is installed at the room side it leads to very low effective heat capacity, irrespective of the wall thermal resistance. Based on the first order results from a simplified room model, it was subsequently found that the influence of these parameters on the effective heat capacity of the building envelope leads to significant effects on the transient thermal behavior, thermal time constant and stability of structures.

Theory and Numerical Modelling of the Effects of Ablator Wall and Hohlraum Transfer Gas Thermal Resistances on Deuterium-Tritium Redistribution Rates

The influence of capsule wall material and the transfer gas surrounding the capsule on the time required for beta-heating-induced redistribution of a 50-50 mole percent mixture of deuterium and tritium (DT) in a spherical capsule are investigated analytically and numerically. The derivation of an analytical solution for the redistribution time in a one-dimensional binary diffusion model, which includes the thermal resistance of the capsule, is first described. This result shows that the redistribution time for a high conductivity capsule wall is approximately doubled after 8 days of 3He formation. In contrast, with a low thermal conductivity capsule wall (e.g., polyimide), the redistribution time would increase by less than 10%The substantial effect of the capsule wall resistance suggested that the resistance to heat transfer from the capsule through the surrounding transfer gas to the hohlraum wall would also influence the redistribution process. This was investigated with a spherical model, which was based on accounting for energy transfer by diffusion with a conduction heat transfer approximation. This made it possible to solve for the continuous temperature distribution throughout the capsule and surrounding gas. As with the capsule the redistribution times depended on the relative values of the thermal resistances of the vapor in the capsule and the transfer gas. With increasing vapor thermal resistance (increased concentration of 3He) redistributions times for hydrocarbon capsules were less than the minimum one-dimensional value of 27 minutes. Further analytical and experimental investigation of the thermal interaction between the capsule and hohlraum is recommended.

Thermal Interaction Between Two Vertical Systems of Free and Forced Convections

Conjugate heat transfer between two thermal systems of free and forced convection, separated by a vertical wall, is investigated theoretically. It is assumed that the heat conduction in the wall is only in the transversal direction. The situation considered is one in which the two vertical boundary layers of free and forced convection induced on wall sides are in counterflow. The study shows that the thermal interaction process between the two thermal systems is controlled by two dimensionless groups: The ζ parameter relates the heat transfer effectiveness of two convection layers, and ω parameter evaluates the wall thermal resistance to the forced-layer resistance. The obtained results explain the effects of these two parameters on the fluid-solid interfacial temperature at wall sides, the temperature fields in the solid and fluid regions, as well as the local and mean Nusselt numbers. Comparison is made between model predictions and numerical results obtained by FLUENT for special model problems.

Wall heat loss from intermittently conditioned spaces—The dynamic influence of structural and operational parameters

The intermittent daily operation of the HVAC equipment is currently a method usually adopted for reasons of proper building energy management in geographical regions with temperate climatic conditions. Under these conditions, the influence of the building envelope and the specific operational schedule of the mechanical equipment under the effect of time-varying ambient temperature and incident solar radiation is responsible for the development of complex transient heat transfer phenomena, which leads to continuous heat exchange with the environment. The heat exchange through the building envelope, which determines its energy efficiency, is strongly influenced by several parameters along with the thermal characteristics of the building elements like the wall thermal resistance and heat capacity as well as the thermal time constant and operational conditions of mechanical equipment. In the present investigation, an analysis is developed for the evaluation of the daily quasi steady-state energy losses from composite walls under the effect of time-varying meteorological driving forces, when heating at the room side is intermittent. The effect of various significant design and system operational parameters like wall heat capacity, thermal time constant and the insulation layer configuration, daily heating cycle duration and phase, as well as room heat capacity and net heating power input are investigated on the quasi steady-state energy loss and daily room temperature swing under typical winter conditions.

Thermal Management in Catalytic Microreactors

In reactors with one or two physical dimensions in the microscale and the remaining dimensions in the mesoscale or macroscale, thermal uniformity is often assumed. However, this assumption is not necessarily correct, especially for well-insulated reactors with fast reaction chemistries. In this paper, a catalytic microreactor with “tunable” wall thermal resistance (conductivity and thickness) is fabricated. Changing the thermal properties of the wall of this reactor is found to have a substantial effect on reactor thermal uniformity but only a slight effect on conversion and extinction limits. Further experiments show that the flow rate and feed composition for this reactor have a large effect on the operating temperature but only a moderate effect on thermal uniformity. An analysis of the overall energy balance of this system indicates that more than 50% of the generated heat is lost to the surroundings before the fluid exits the reactor, although this energy exchange becomes less efficient as the flow r...

Use of a phase change material to prevent frosting in a compact crossflow air exchanger

A mathematical model is presented for simulating the thermal behaviour of a cross-flow air exchanger with layers of phase change material (PCM) sandwiched between the hot and cold air streams to prevent frosting. The exchanger operates under winter conditions and uses electric power for storing heat in the PCM. The model is validated with experimental data, and the effects of the dimensionless electric load, Q e, and of the PCM layer Biot number, Bi, are examined. Results indicate that for a commonly used exchanger with Bi=0.6 (3 mm PCM layer) and Q e⩾0.234 (300 W/m 2), frosting is avoided during both the charge and discharge periods. For Bi=1.2 (6 mm PCM layer), Q e may be set to zero (no storage) and frosting is still avoided, but the increased wall thermal resistance affects the exchanger performance by reducing the dimensionless outlet temperature of the cold air stream.

Evaluation of heat transfer in a catalytic fixed bed reactor at high temperatures

Experimental results of fixed-bed heat-transfer experiments with no chemical reaction are presented and discussed. The runs were carried out in a tubular integral reactor heated by an electrical furnace at temperatures in the range of 100 to 500°C. Experimental temperature profiles were determined for the electrical furnace, for the reactor wall, and for the fixed bed center. Industrial catalyst for the prereforming of hydrocarbons was employed as the packing material. The effects of process conditions (furnace temperature, gas flow rate) on the heat-transfer coefficients were evaluated. The experimental results were analyzed in terms of the external, wall, and internal thermal resistances, associated in series, and compared with model predictions. Under the conditions studied, the overall coefficient was mostly a function of the external effective heat-transfer coefficient. An alternative data treatment was proposed to determine the internal heat-transfer coefficient in fixed beds when wall temperature is not constant.

Optimizing insulation thickness for buildings using life cycle cost

A systematic approach for optimization of insulation material thickness is developed in this paper and then applied to Palestine. The optimization is based on the life cycle cost analysis. Generalized charts for selecting the optimum insulation thickness as a function of degree days and wall thermal resistance are prepared. Life cycle savings of the insulated buildings are computed for Palestine. Savings up to 21 $/m 2 of wall area are possible for rock wool and polystyrene insulation. Payback periods between 1 and 1.7 years are possible for rock wool and payback periods between 1.3 and 2.3 years for polystyrene insulation, depending on the type of wall structure.

The effects of temperature conditions on the thermal resistance of walls made with different shapes vertical hollow bricks

The authors used a testing program in order to determine the thermal resistance of terra-cotta walls; built with vertical hollow bricks. These bricks could be standard ones, so cavities were 25 cm high, or they could reach 250 cm (story-height bricks). Different hollow shapes were tested under different boundary conditions on temperature. The authors tried to determine for this class of bricks used in France, the most performing outline, from a thermal point of view, under different boundary conditions.

高速実験炉「常陽」におけるオンライン計装照射技術

In order to obtain precision irradiation data, various on-line instrumented irradiation test are required. Since 1985, these tests have been performed in the experimental fast reactor JOYO. The Oarai Engineering Center (OEC) has put recent years of large effort for the development of the Material Testing Rig with Temperature Control (MARICO) and the Ex-vessel Irradiation Rig (EXIR) in JOYO. Both these are successfully able to control their irradiation condition during reactor operation.The MARICO was designed mainly for in-core biaxial creep rupture testing of fuel cladding within a controlled temperature of ±4°C. In order to adjust the capsule wall's thermal resistance for the capsule temperature control, changing mixture ratio of argon gas and helium gas in the capsule's double wall is necessary. The EXIR, which is installed adjacent to the reactor vessel, was designed for uniaxial creep testing of reactor structure materials using an electric heater and stress control system. The stress control system is composed of an argon gas actuate bellows mechanism and differential transducer instrument.

Thermal interaction between laminar film condensation and forced convection along a conducting wall

A theoretical analysis is presented to investigate the thermal interaction between laminar film condensation of a saturated vapor and a forced convection system separated by a heat conducting wall. In this work, the effect of the wall thermal resistance is considered. It is assumed that the countercurrent boundary layer flow is formed on the two sides. Governing boundary layer equations together with their corresponding boundary conditions for film condensation and forced convection are all cast into dimensionless forms by using the non-similarity transformation. The resulting system of equations is solved by using the local non-similarity method in conjunction with the fourth order Runge-Kutta method in conjunction with the Nachtsheim-Swigert iteration scheme. The total heat flux through the wall and the wall temperature distribution are determined. The present results show that the effect of the forced convection Prandtl number Pr c is not negligible for large values of the thermal resistance ratioA*, and the effect ofA* and Pr c on the overall heat transfer through the wall is more pronounced than that of the Jakob number and film Prandtl number.