Wall Thermal Conductivity Research Articles

Effects of Wall Properties on Temperature-Control Effectiveness of Heating Section in a Thermosiphon Containing PCM Suspensions

This article considers the problem of natural heat transfer in a rectangular thermosiphon to investigate the effects of wall properties (thickness and thermal conductivity) on the heat-transfer characteristics of phase-change-material (PCM) suspension flow. The following parameter ranges were investigated: dimensionless loop-wall thickness, 0–0.5; wall-to-fluid thermal-conductivity ratio, 0.1–100; modified Rayleigh number, 1010–1011; and volumetric fraction of PCM particles, 0–10%. From numerical simulations via the finite-volume approach, it was found that using a pipe with appropriate wall thickness and thermal conductivity containing PCM suspensions for the heating section of a rectangular thermosiphon can effectively control the maximal temperature.

Effects of fuel composition and wall thermal conductivity on thermal and NOx emission performances of an ammonia/hydrogen-oxygen micro-power system

As a renewable fuel, ammonia NH3 is identified as one of the most potential candidates to tackle greenhouse gas challenge. For this, ammonia-hydrogen-oxygen combustion in micro-power systems is numerically investigated in this work. The computational model is first validated with experimental data available in the literature. Then, emphasis is placed on the effects of (1) the fuel composition ratio ε (defined as the hydrogen molar fraction relative to the mixed fuel of ammonia and hydrogen) and (2) the wall thermal conductivity (WTC) on the system's working performance. Results indicate that increasing ε can not only lead to the outer wall temperature (OWT) and radiation efficiency (RE) being decreased, but also enable the flame propagate towards the combustor inlet. An optimized ε is found to minimize the NOx emission by approximately 54.9% compared to 100% NH3 combustion. Further analyses show that a high WTC has a potential to increase OWT at low fuel flow rates, while RE varies non-monotonically with the NH3 volumetric flow rate. It is also shown that WTC has a slight effect on NOx emission and the flame shape. This work sheds light on a simple but effective way to improve thermal and emission behaviors.

Mathematical Models for Determining the Distribution of Fluid Flow Temperature along the Wellbore and Horizontal Pipeline

This paper presents a proposed new indirect method determining instantly oil well debit using developed mathematical models. As a result integrated analysis using the models it has been revealed correlation between oil well debit and well throw out flow temperature. Therefore putting purpose was obtained. Mathematical models are developed for the distribution of fluid flow temperature along the length of the tubing from the well bottom to the wellhead and along the length of the oil pipeline from the collector of oil wells to the oil treatment unit. On the basis of experimental data, the authors propose formulas in the form of the relationship between oil emulsion (OE) viscosity, the flow temperature and concentration of water globule in OE and the coefficient of heat transfer from the fluid flow in the wellbore (WB) to the rock, and heat capacity and thermal conductivity of gas, water, rock and steel of the WB walls. This effect is demonstrated in the constructed diagrams. It is shown bottom temperature jump as a result of the Joule Thomson drosseling effect then connective transmitted up at flow rate v. In such case well-head or well outlet oil mixture (OM) flow temperature depend more of volume of stream flow than of bottom hole temperature. Thought in the paper, do not taking into consideration great casing annulus areas influence to the well outlet flow temperature. As shown from supporting paper the relative values og the thermal conductivity of the liquid column and gas column present in the casing annulus order less than well bore (WB) wall thermal conductivity. Consequently well outlet OM flow temperature will depends not only of the volume of stream flow, also of the bottom hole temperature and of the gas column and liquid column.A new method for determining the oil well flow rate by measuring the downstream temperature is developed. A mathematical model is proposed that allows calculating the thermal profile of the fluid along the wellbore for determining the oil well flow rate with account of the geothermal gradient in the rock surrounding the wellbore. It is shown, that unlike the existing methods the new proposed method allows determining the instantaneous discharge of a well very easily. One of the actual challenges in fluid (oil, water and gas) transportation from wells to oil treatment installation is determination of a law of temperature distribution along the length of a pipeline at low ambient temperature. That temperature leads to increase in viscosity and deposition of wax on inner surface of a pipe. To overcome that challenge it is needed to consider several defining characteristics of formation fluid (FF) flow. Complexity of a solution is caused by two factors. From the one hand, in most cases (especially on a late stage of field development) FF is an oil emulsion (OE) that contains gas bubbles. From the other hand, temperature gradient between fluid flow and the environment has significant value (especially in the winter period of the year). At the same time, the higher content of emulsified water droplets (EWD) in OE and lower flow temperature, the higher FF viscosity, and consequently productivity (efficiency) of oil pumping system is reduced. Performed research and analysis of field experimental data showed that a function of oil viscosity versus temperature has a hyperbolic law; a function of OE viscosity versus concentration of EWD has a parabolic one. A heat balance for a certain section of a pipeline in steady state of fluid motion using a method of separation of variables was established taking into account above mentioned factors, Fourier’s empirical laws on heat conductivity and Newton’s law on heat transfer. As a result, unlike existing works, an exponential law of distribution of temperature along the length of a pipeline is obtained. A law takes into account nonlinear nature of change in viscosity of OE from change in temperature of flow and concentration of water in an emulsion. As a result, in contrast to the existing works, the proposed exponential law of temperature distribution along the length of the pipeline is obtained, taking into account the nonlinear nature of variation of OE viscosity with the change in the flow temperature and the concentration of water in the emulsion. The results of the calculation are presented in the form of a table and graphs.

Conjugate Heat Transfer Analysis of PCM Suspensions in a Circular Tube under External Cooling Convection: Wall Conduction Effects

In this paper, a numerical method is used to investigate the conjugate heat transfer of a phase change material (PCM) suspension in a circular tube under external cooling convection. The following parameters and ranges were considered: dimensionless tube wall thickness, t w (0–0.5); wall-to-fluid thermal conductivity ratio, k w f * (0.1–10); volumetric fraction of PCM particles, c v (0.1); Biot number, B i o (1); Stefan number, Ste (0.1); and Peclet number, Pe (1000). The results show that the wall thermal conductivity considerably affects the outer/inner wall temperature of the tube, the average temperature of the working fluid, and the volumetric liquid fraction of PCM particles. Thus, wall conduction effects must be properly accounted for to model heat transfer in a PCM suspension in tube flow.

Numerical simulations of a double-wall cooling with internal jet impingement and external hexagonal arrangement of film cooling holes

Three-dimensional conjugate computations are conducted to investigate the flow and heat transfer characteristics of a double-wall cooling system with a novel hexagonal arrangement of film cooling holes. Under the same coolant flow rate, the heat transfer and cooling performance of angled holes in the hexagonal arrangement are investigated and compared with those in the counterparts with conventional linear arrangements of external film cooling holes. The boundary condition of numerical simulations maintains the fixed jet Reynolds numbers of 10,000, 20,000 and 30,000 and the corresponding blowing ratios of 0.5, 1.0 and 1.5. Different wall thermal conductivities are respectively used in the numerical computations in order to examine the effects of Biot number of the film cooling wall on the overall cooling performance. Polyhedral meshes and Shear-Stress Transport (SST) k−ω turbulence model have been adopted in the numerical computations to obtain the detailed heat transfer and flow field. The results indicate that the hexagonal arrangement of film cooling holes shows significant advantages for the double-wall cooling scheme to decrease the external surface temperature appreciably. As the Biot number increases the overall cooling effectiveness of the double-wall system shows a downward trend, which decreases by about up to 25% in the Biot number range of 0.3–1.2. With the engine-like turbine Biot number of Bi≈0.56 the hexagonal arrangement improves the total heat transfer quantity inside the film cooling holes by up to 3.8 times, and enhances the overall effectiveness by up to 24.5%, as compared with the linear arrangement of straight circular film cooling holes. The combinations of the increased convective heat transfer inside the film cooling holes and the improved external film coverage induced by the hexagonal arrangement are the main contributions to the improvement of overall cooling performance.

CFD investigation on the characteristics of annular pulse tube

Abstract CFD models are used to study the performance of circular and annular pulse tubes working at 77–300 K. The oscillating flow and heat transfer characteristics are investigated. In terms of pulse tube impedance, simulation results indicate that the annular shape has negligible influence when the length and flow area are kept the same as circular pulse tube. However, compared with the circular pulse tube, the thermal performance of annular pulse tube apparently deteriorates which is mainly due to the increased thermal losses caused by heat transfer between the internal gas and two cylindrical tube walls (instead of one in the case of circular pulse tube). The expansion efficiency (defined as the enthalpy flow divided by acoustic power at the cold end) of annular pulse tube is lower by about 14% compared with that of circular pulse tube. Among the influencing factors, hydraulic diameter of annular pulse tube has a significant influence on the performance, and smaller hydraulic diameter leads to a reduced expansion efficiency. The effects of tube wall thermal conductivity are also discussed. This study provides a better understanding on the mechanism of annular pulse tube which sets the basis for optimizing a compact low temperature cryocooler for future applications.

Multiscale approach to model steady meniscus evaporation in a wetting fluid

Evaporation flux in the thin transition film close to the wall is up to 100 times greater than at the center of a wetting meniscus. Despite higher evaporation flux, the thin film accounts for a relatively small part of the total evaporation from the liquid-vapor interface. The thin-film contribution is directly proportional to the solid wall thermal conductivity and inversely proportional to vapor pressure and Bond number.

Numerical Simulations on the Application of a Closed-Loop Lake Water Heat Pump System in the Lake Soyang, Korea

A lake is one of the geothermal energy sources to meet increasing demands for renewable energy use. In this study, a series of numerical modeling was performed to evaluate the applicability of a close-loop lake water heat pump (LWHP) system in Lake Soyang, Korea. A non-isothermal pipe flow model was used to simulate the flow and heat transfer processes occurring in the LWHP system with the main pipe and several helical tubes for heat exchange. Based on the temperature data measured in the Lake Soyang for 4 years, the installation depth and the number of helical tubes were determined sequentially, and the sensitivities of additional installation and operation factors on the system performance were analyzed. Assuming a mild current in the lake, the installation and operation conditions for the efficient operation of the system were suggested as follows: The installation of 16 helical tubes at 50 m deep, the circulation rates of heat-carrier fluid of 189.3 L/min, the inner diameter of tubes of 32 mm, and the wall thickness and thermal conductivity of 2.9 mm and 0.4 W/mK, respectively. Considering many lakes and reservoirs in Korea, the closed-loop LWHP system would be a viable renewable energy application.

Viscoelastic dream pipe with triangular pressure gradient wave form for substantial enhancement of heat transfer

A theoretical investigation is carried out to analyse the combined effects of oscillation and conjugation on the enhancement of heat transfer in laminar flow of viscoelastic fluids (CPyCl/NaSal) flowing in a circular tube with thermally conducting walls. The fluid flow is induced by a triangular pressure gradient wave form. Closed-form solutions for the momentum and heat equations are presented. The effects of influential parameters such as the wall thermal conductivity (ks), wall thickness (ε), fluid thickness, Womersley number (α), Deborah number and Prandtl number on the effective thermal diffusivity (κe) are examined. Several maxima occur in κe where the fluid flow exhibits a resonant behavior. In the viscous regime, κe increases with increasing ks and ϵ only up to the optimum value of α. In the elastic regime, the effect of conjugation on κe is saturated. The highest heat flux is achieved by the viscoelastic fluids with sinusoidal pressure gradient wave form when the walls are thermally insulated and this maximum heat flux (6.53 × 1011 W/m2) is 2968 times higher than the next highest value (2.2 × 108 W/m2) achieved by the Newtonian fluids with triangular pressure gradient wave form with thermally conducting walls. The present study demonstrates that viscoelastic fluids are preferable to Newtonian fluids and further triangular pressure gradient wave form is preferable when the walls are thermally conducting and sinusoidal pressure gradient wave form is preferable when the walls are thermally insulated.

Combustion characteristics and radiation performance of premixed hydrogen/air combustion in a mesoscale divergent porous media combustor

To improve flammability and radiation efficiency, a divergent porous media combustor is proposed and numerically studied. The local thermal non-equilibrium model is used to consider the temperature difference between gas and solid matrix. Effects of equivalence ratio, the wall thermal conductivity, solid matrix thermal conductivity, and divergent ratio on combustion characteristics, radiation efficiency, and flammability limits are studied. The results show that the divergent channel extends the blowout limit by 186% and obtains a maximum radiation efficiency of 29.3%, increased by 70% compared with the straight channel. A smaller wall thermal conductivity is recommended considering the flammability range and radiation efficiency. A careful choice of solid matrix thermal conductivity and the divergent ratio is suggested to balance their opposing effects on the radiation efficiency and the flammability.

A numerical study on premixed hydrogen/air flames in a narrow channel with thermally orthotropic walls

Premixed hydrogen/air micro-flame stabilizations in a narrow channel confined by two parallel plates with thermally isotropic and orthotropic wall materials are numerically studied using a OpenFOAM-based, reacting flow code. For a range of simulated equivalence ratios and inflow velocities, two modes of flame shapes (convex-shaped and concave-shaped) are observed, accompanying with variations of the number of heat release rate peaks in flame structures, which can be attributed to the appearance of some critical O-participating and H-participating elementary reactions. Flame stability limits are studied for three sets of wall thermal conductivities of k = 16 W/m K, k = 128 W/m K (isotropic) and kxx = 128 W/m K &kyy = 16 W/m K (orthotropic). The low velocity limits show invariant with wall thermal conductivities, while the high velocity limits in descending order are found to be: “k = 128 W/m K” > “kxx = 128 W/m K &kyy = 16 W/m K” > “k = 16 W/m K”. The logic behind is the competition between two mechanisms: the wall pre-heating effects and the transverse heat losses to the ambient. The critical convective heat transfer coefficients that reflect the combustor's ability to resist heat losses are also investigated among the three cases. The reduction of the transverse thermal conductivity can have a high critical coefficient value in the low-inflow velocity regime while makes negligible impacts on extending the critical coefficient in the high-inflow velocity regime. In summary, the use of thermally orthotropic wall materials leads to a slightly decreased high velocity limit (~3% lower) but a considerably increased critical convective heat transfer coefficient in the high-inflow velocity-regime (~25% higher), as compared to the thermally isotropic combustor of k = 128 W/m K.

Flame dynamics and stability of premixed methane/air in micro-planar quartz combustors

The flame dynamics and stability of methane/air premixed combustion in micro-planar quartz combustors were numerically studied with detailed chemical reaction mechanism. The computed flame characteristics were quantitatively and qualitatively in good agreement with experimental phenomena. The oscillating flame was observed from the perspective of three-dimensional simulation for the first time. Parametric analysis associated with channel height and channel length was performed. Results indicated that the blowout limit was found to increase first and then decrease as increasing the channel height due to the tradeoff relationship between increased areas for heat flux and the extended timescales for energy diffusion. The change in channel length can lead to the variation in residence time of mixture in channel and heat loss from the external wall. It was found that there existed a critical point of channel length, above which the flame stability was improved marginally. Furthermore, it was shown that wall thermal conductivity had effect on both the flame structure and flame location, especially at relatively high inlet velocities. Further analyses indicated that the mechanism of enhanced flame stability with sufficient high wall thermal conductivity can be attributed to the lower strain rate and smaller deflection of streamlines in flame front.

Thermal management of methanol reforming reactors for the portable production of hydrogen

Three-dimensional numerical simulations were performed to address the thermal management issues associated with the design of a methanol reforming microchannel reactor for the portable production of hydrogen. The design of the reactor was fundamentally related to the direct coupling of reforming and combustion reactions by performing them on opposite sides of dividing walls in a parallel flow configuration. Effective autothermal operation was achieved through a combination of microchannel reactor technology with heat exchange in a direction perpendicular to the reacting fluid flow. Computational fluid dynamics simulations and thermodynamic analysis were carried out to investigate the effect of various design parameters on the characteristics of the generation, consumption, and exchange of thermal energy within the system. The results indicated that the ability to control temperature and temperature uniformity is of great importance to the performance of the system. The degree of temperature uniformity favorably affects the autothermal operation of the reactor. Temperature uniformity of the reactor can be improved by controlling the rate of heat transfer through a variety of factors such as wall thermal conductivity, fluid velocities, and dimensions. High wall thermal conductivity would be greatly beneficial to the performance of the system and the temperature uniformity of the reactor.

Multi-factor impact mechanism on combustion efficiency of a hydrogen-fueled micro-cylindrical combustor

As it is important to achieve higher combustion efficiency for applications of micro-cylindrical combustor, the multi-factor impact mechanism on the combustion efficiency of a hydrogen-fuelled micro-cylindrical combustor is investigated in this work. Firstly, six factors such as hydrogen/air equivalence ratio, inlet velocity, inlet temperature, wall thermal conductivity, wall emissivity and convective heat transfer coefficient of outer wall and five levels of each factor are determined. Orthogonal design table L25(56) is introduced to arrange cases. Secondly, grey relational analysis is adopted to investigate the effects of the six factors on combustion efficiency. Finally, the results of grey relational analysis are validated by analysis of variance. Based on grey relational analysis and analysis of variance, it is determined that the impact ranking from the largest to the smallest is hydrogen/air equivalence ratio, inlet velocity and inlet temperature, followed by the other three factors. The impact of wall thermal conductivity, convective heat transfer coefficient of outer wall and wall emissivity is considered to be equal due to their difference of impact on combustion efficiency is very small. This work provides us significant reference for optimizing combustion efficiency of a hydrogen-fuelled micro-cylindrical combustor.

Influence of conjugate convective heat transfer on temperature fields in thin walls that organize liquid layers of various orientations

Numerical studies of unsteady conjugate natural convective heat transfer in the model of a thin-walled fuel tank are carried out in the conjugate formulation. The fields of temperature in liquid and in solid walls of the tank are calculated. The evolution of convective flows and temperature fields after a sudden supply of heat to the base of the tank is investigated. Flat layers of liquid enclosed between flat walls of different orientations with inclination angles of 0, 30, 45, 60 and 90 degrees are considered. With the angle of inclination of 0 degrees the outer surface of the side walls is insulated. The outer surface of the lower wall heats up suddenly, the outer surface of the upper wall is isothermally cold. The system of equations of thermogravitation convection in the Boussinesq approximation in dimensionless form, written in terms of temperature, vector potential of the velocity field and velocity vortex, is solved. It is shown that an inhomogeneous temperature field is formed inside the solid walls of the finite thermal conductivity. The thermal conductivity of walls and conditions at the upper boundary of the liquid layer affect significantly the spatial form of convective fuel flows in fuel tanks and laws of conjugated natural convective heat exchange.

Effect of Channel Diameter on the Combustion and Thermal Behavior of a Hydrogen/Air Premixed Flame in a Swirl Micro-Combustor

Improving the flame stability and thermal behavior of the micro-combustor (MC) are major challenges in microscale combustion. In this paper, the micro combustions of an H2/air premixed flame in a swirl MC with various channel diameters (Din = 2, 3, 4 mm) were analyzed based on an established three-dimensional numerical model. The effects of hydrogen mass flow rate, thermal conductivity of walls, and the preferential transport of species were investigated. The results indicated that the flame type was characterized by the presence of two recirculation zones. The flame was anchored by the recirculation zones, and the anchoring location of the flame root was the starting position of the recirculation zones. The recirculation zones had a larger distribution of local equivalence ratio, especially in the proximity of the flame root, indicating the formation of a radical pool. The combustion efficiency increased with an increasing Din due to the longer residence time of the reactants. Furthermore, the MC with Din = 2 mm obtained the highest outer wall temperature distribution. However, the MC with Din = 4 mm had a better uniformity of outer wall temperature and large emitter efficiency due to the larger radiation surface. An increase in thermal conductivity boosts the thermal performance of combustion efficiency, emitter efficiency, and wall temperature uniformity. But there is a critical point of thermal conductivity that can increase the thermal performance. The above results can offer us significant guidance for designing MC with high thermal performance.

Practical Jointed Approach to Thermal Performance of Functionally Graded Material Annular Fin

Using the complementary functions method, a general solution for the one-dimensional steady-state heat conduction equation for the fin together with the boundary conditions made of functionally graded material is presented. Two cases for the variation of the functionally graded material wall thermal conductivity were considered. These are 1) power and 2) exponential types of variation of the thermal conductivity varying with the radial coordinate. In the present paper, a semi-analytical iterative technique, one of the most efficient unified methods, is employed to solve the heat conduction equation. For different values of the inhomogeneity constant, distributions of temperature, heat transfer rate, fin efficiency, and fin effectiveness, as a function of radial direction, are obtained. Various material models from the literature are used and corresponding temperature distributions are computed. Verification of the proposed method is done using benchmark solutions available in the literature for some special cases, and virtually exact results are obtained.

Change in conductivity of magnetohydrodynamic Darcy–Forchheimer second-grade fluid flow due to variable thickness surface

The current investigation is communicated to analyze the characteristics of Darcy–Forchheimer second-grade fluid flow enclosed by a deformable sheet in the existence of both variable thermal conductivity and magnetohydrodynamics. The leading nonlinear energy and momentum partial differential equations are converted into nonlinear ordinary differential equations by utilizing suitable analogous approach. Then the acquired nonlinear problem is numerically calculated by utilizing BVP4C (built in) technique in MATLAB. The influence of certain appropriate physical parameters, namely wall thickness, second-grade fluid, Hartmann number, power index, porosity parameter, inertia coefficient, Prandtl number, and thermal conductivity, on temperature and velocity is studied and deliberated in detail. Numerical calculations of Nusselt number and skin friction for distinct estimations of appearing parameters are analysed through graphs and tables.

Differential Global Reaction Model with Variable Stoichiometric Coefficients for Thermal Cracking of n-Decane at Supercritical Pressures

This study presents a new modeling approach for the thermal cracking of hydrocarbon fuels at supercritical pressures. The thermal cracking process is treated as an infinite number of continuous microreactions at different fuel conversions, in which the stoichiometric coefficients of each species are expressed by continuous differentiable functions of the fuel conversion to consider the effects of secondary reactions. A set of thermal cracking experimental results involving n-decane was used as an example to show how to establish the differential global reaction (DGR) model with variable stoichiometric coefficients. The DGR model was implemented in a computational fluid dynamics simulation to predict n-decane thermal cracking coupled with flow, heat transfer, and wall thermal conduction. The results showed that the DGR model can more accurately predict the species mass fractions than existing global reaction models, especially for conditions when the secondary reactions strongly impact the thermal cracking...

Thermal management in catalytic heat-recirculating micro-combustors: A computational fluid dynamics study

The characteristics of heat transfer in catalytic heat-recirculating micro-combustors were studied numerically to provide an effective thermal management method for small-scale combustion systems. Particular emphasis is placed on understanding the role of wall thermal properties in determining the stability of combustion in small-scale catalytic heat-recirculating systems. A computational fluid dynamics model was developed to solve thermal management problems involving combustion for such systems. Numerical simulations were performed to provide in-depth understanding of heat transfer characteristics and to evaluate the benefits associated with the heat-recirculating design. The optimal conditions were determined for stable and safe operation. Recommendations for the design of small-scale heat-recirculating systems were presented to satisfy the requirements of combustion and materials in terms of stability. The results indicated that the wall thermal properties of a heat-recirculating system play a significant role in its thermal uniformity and combustion stability. The choice of design for the system depends on its wall thermal conductivity. Moderate wall thermal conductivities are preferable for improving thermal uniformity and combustion stability while eliminating hot spots. Excellent thermal uniformity can be achieved by using highly conductive materials at the expense of the stability of combustion. Highly insulating materials present significant challenges in the stability of both combustion and materials. Engineering maps were finally constructed for the system in terms of both efficiency and safety.