External Convective Heat Transfer Research Articles

The Thermal Characteristics of a Helical Coil Heat Exchanger for Seawater-source Heat Pump in Cold Winter

A high-density polyethylene helical coil heat exchanger (HCHE) is firstly adopted by a seawater-source heat pump system (SWHP) and experiments are conducted to study the thermal performance of the heat exchanger in icy condition. The external convective heat transfer coefficient is calculated by the experimental results and used in the development of the mathematical models. To predict the heat transfer process of the HCHE in icy condition in frigid periods of winter, a mathematical model is developed. Simulations with different parameters are conducted to investigate the effects of variable parameters on the thermal performance, such as inlet temperature, intermediate medium's flow rate, heat exchanger's length and diameter, temperature of seawater. The study indicates that the developments of the mathematical model are very helpful in the designing of heat exchangers used for the SWHP system.

Overall Effectiveness and Flowfield Measurements for an Endwall With Nonaxisymmetric Contouring

Endwall contouring is a technique used to reduce the strength and development of three-dimensional secondary flows in a turbine vane or blade passage in a gas turbine. The secondary flows locally affect the external heat transfer, particularly on the endwall surface. The combination of external and internal convective heat transfer, along with solid conduction, determines component temperatures, which affect the service life of turbine components. A conjugate heat transfer model is used to measure the nondimensional external surface temperature, known as overall effectiveness, of an endwall with nonaxisymmetric contouring. The endwall cooling methods include internal impingement cooling and external film cooling. Measured values of overall effectiveness show that endwall contouring reduces the effectiveness of impingement alone, but increases the effectiveness of film cooling alone. Given the combined case of both impingement and film cooling, the laterally averaged overall effectiveness is not significantly changed between the flat and the contoured endwalls. Flowfield measurements indicate that the size and location of the passage vortex changes as film cooling is added and as the blowing ratio increases. Because endwall contouring can produce local effects on internal cooling and film cooling performance, the implications for heat transfer should be considered in endwall contour designs.

A novel energy simulation approach for thermal design of buildings equipped with radiative panels using inverse methodology

The purpose of this research is to present a novel approach to estimate thermal design parameters with unknown values in a building equipped with radiative panels with the use of inverse methodology. A direct model is selected to analyze thermal behavior of building components in order to determine indoor temperature field using direct numerical modeling. Using inverse analysis, three thermal design parameters with unknown values, such as conductivity, internal and external convection and total heat transfer coefficients, are estimated using Levenberg–Marquardt and conjugate gradient methods with the knowledge of temperature field resulted from direct modeling. To illustrate the sufficiency of inverse technique, the results of both noisy and non-noisy estimations are presented using sensitivity analysis. To verify the accuracy of the present study, the results of inverse analysis are compared with those of numerical simulation presented in the literature, which indicates a good agreement between numerical and inverse solutions. It is shown that inverse analysis can be used as an applicable approach to increase computational precision in building thermal design phase.

An adapted steady RANS RSM wall-function for building external convection

Computational Fluid Dynamics (CFD) can improve usual estimates of building external convective heat transfer coefficients (hc,w) by accounting for the geometry of constructions, the aerodynamic field around them, the nature of convection and providing high resolution data. However, the limitations of usual mass flow descriptions and near wall treatments make the accurate prediction of hc,w challenging. Hence, this paper evaluates the ability of steady RANS Reynolds Stress (RSM) and k–ε realizable models to predict hc,w in case of isolated cubical obstacles. The accuracy of usual CFD methods and turbulence models, as well as fine grid near wall models and usual temperature wall functions (TWFs) are examined by comparison with experimental and detailed numerical data.When used with a low Reynolds number model (LRNM), both turbulence models accurately predict hc,w on the front and rear faces of the obstacle. However, they show different behaviors on the other faces and highlight issues related to the dynamic behavior of real flows. Moreover, hc,w predictions obtained using standard TWFs substantially deviate from the validated LRNM results. Therefore, a customized TWF suited for use with the RSM and forced convection problems is proposed by extending studies of Defraeye et al. (An adjusted temperature wall function for turbulent forced convective heat transfer for bluff bodies in the atmospheric boundary layer; Building and Environment, 2011, 46, 2130–2141). Customized TWFs substantially improve WF-based hc,w predictions with respect to LRNM results while keeping their cost effectiveness, and provides satisfactory results even for high z∗.

Numerical investigation of heat transfer characteristic of fixed planar elastic tube bundles

Planar elastic tube bundles are a novel approach to enhance heat transfer by using flow-induced vibration. This paper studied the heat transfer characteristic and fluid flow in both tube-side and shell-side using numerical simulation. Two temperature difference formulas were used to calculate convective heat transfer coefficient and the results were verified by theoretical analysis and experimental correlations. The effect of Reynolds number on overall convective heat transfer coefficient and pressure drop in tube-side and shell-side were studied. The comparison of the secondary flow in planar elastic tube bundles and conical spiral tube bundles were conducted. The external flow field and local convective heat transfer around the periphery of fixed planar elastic tube bundles subjected to the cross fluid flow were also analyzed. The results show that the energy consumption efficiency should be taken into account in the forced heat transfer process conducted by adjusting the fluid flow. The secondary flow varies depending on the fluid flow state and the geometry of tube. Hence, it is deduced that the heat transfer enhancement is obtained because the thermal boundary layer in the deformed planar elastic tube bundles caused by flow-induced vibration is damaged by the disordered secondary flow. In addition, the convective heat transfer capability of outside the two intermediate tube bundles is enhanced because of the effect of irregular and complex fluid flow affected by the role of curved tubes on both sides.

Axisymmetric Contact Problem of Thermoelasticity for a Three-Layer Elastic Cylinder with Rigid Nonuniformly Heated Core

We consider an axisymmetric contact problem for a finite three-layer elastic cylinder with rigid nonuniformly heated core under the conditions of external convective heat transfer. The conditions of perfect unilateral mechanical contact and imperfect thermal contact are given on the surfaces of possible contact of the cylinder with the core and between the layers. The variational formulation of the problem is obtained. Finally, we propose an iterative numerical algorithm in each iteration of which it is necessary to solve the uncoupled elasticity and heat-conduction problems and the finite-element method is used for their discretization.

Numerical Simulation of Convective-radiative Coupled Heat Transfer Performance for High Altitude Airships

In order to understand thermal characteristics of high altitude airships, the numerical simulation of convective-radiative coupled heat transfer performance at a certain time in a day is carried out using Ansys Fluent software. The governing equations of flow are using the compressible Reynolds averaged N-S equations and standard κ-ɛ turbulence model. A non-uniform temperature distribution boundary condition in the complex thermal environment and heat transfer mechanism is applied on the surface of high altitude airships based on user definition function (UDF) programming. The paper then simulates the external convection heat transfer characteristics at various flow conditions at 40 degrees north latitude at the altitude about 20km at noon in the summer solstice. Finally, the corresponding calculation results of the surface temperature distribution, the flow field distribution and the convective heat flux distribution of airship envelope are presented, which are also analyzed in this paper.

Natural Convection from Vertical Parallel Plates: An Integral Method Solution

Steady-state external natural convection heat transfer from isothermal, vertically mounted rectangular fins is modeled analytically. An integral technique is used to solve the governing equations. Compact relationships are developed for the velocity and temperature profiles for the buoyancy-driven channel flow for the range. Comprehensive numerical and experimental studies are performed. The proposed analytical model is successfully validated against the numerical data with the maximum relative differences of 9.8 and 3.5% for velocity and temperature profiles, respectively. Also, new semianalytical local and average Nusselt numbers are reported as functions of the Rayleigh number, temperature difference, and fin aspect ratio; and they are compared against the experimental data with good accuracy of 1.6% maximum relative difference.

Natural Convection From Interrupted Vertical Walls

Steady-state external natural convection heat transfer from interrupted rectangular vertical walls is investigated. A systematic numerical, experimental, and analytical study is conducted on the effect of adding interruptions to a vertical plate. Comsol multiphysics is used to develop a two-dimensional numerical model for investigation of fin interruption effects on natural convection. A custom-designed testbed is built and six interrupted wall samples are machined from aluminum. An effective length is introduced for calculating the natural convection heat transfer from interrupted vertical walls. Performing an asymptotic analysis and using a blending technique, a new compact relationship is proposed for the Nusselt number. Our results show that adding interruptions to a vertical wall can enhance heat transfer rate up to 16% and reduce the weight of the fins, which in turn, lead to lower manufacturing and material costs.

Experimental investigation on the steady, external laminar mixed convection heat transfer characteristics around a large diameter horizontal rotating cylinder

In order to investigate steady, external laminar mixed convection, mainly the correlations about the heat transfer and the critical Reynolds number around a horizontal rotating isothermal cylinder, a particularly manufactured micro-thermocouple is employed to measure the temperature distribution in the thermal boundary layer around the cylinder surface at different rotational speeds, n. The results indicate that there exists a critical Reynolds number Rer,cri as the mean Nusselt number Nu¯ varies with the rotational Reynolds number Rer. The rotation of the cylinder can lead to a nonuniform distribution of local Nussel number Nuφ, and the phenomenon becomes more obvious as Rer exceeds Rer,cri. Nu¯ changes according to a certain rule with Rer. As Rer<Rer,cri, the rotation has little impact on Nu¯, and the free convection heat transfer plays a dominant role, but as Rer≥Rer,cri, Nu¯ increases with increasing Rer, and the heat transfer characteristics are influenced by both the natural and the forced rotational convections simultaneously. Based on the experimental data, the correlations about the heat transfer can be expressed as Nu¯=0.530.0018Rer2.66+Gr⋅Pr0.25, and the critical Reynolds number Rer,cri can be determined by Rer,cri=3.05(Gr⋅Pr)0.456.

Surface shape design in fluid flow problems via hybrid optimization algorithms

Shape design problems, in general, and inverse design problems, in particular, are often solved via optimization techniques. Evolutionary algorithms provide robust and efficient solution methods for such problems. This paper focuses on the application of genetic algorithms (GA), particle swarm optimization (PSO), and two hybrid variants of GA and PSO. Optimum shapes in five shape design problems are found by the proposed hybrid algorithms. Potential, Euler and both laminar and turbulent Navier–Stokes flow solvers are employed in the test problems which include internal and external flows and convection heat transfer. Computational results show that hybridization of GA and PSO improves the convergence rate in all test cases. Up to 30% speed up is observed in the numerical test cases when the hybrid methods are employed and it is also shown that hybrid methods find a better solution in the design space as compared to either GA or PSO.

Natural convection from rectangular interrupted fins

Steady-state external natural convection heat transfer from vertically-mounted rectangular interrupted fins is investigated numerically and experimentally. FLUENT software is used to develop a 2-D numerical model for investigation of fin interruption effects. A custom-designed testbed was developed and 12 different aluminum alloy heatsinks with various geometric dimensions are machined and tested to verify the theoretical results. A comprehensive experimental–numerical parametric study is performed to investigate the effects of fin spacing, and fin interruption. Our results show that adding interruptions to vertically-mounted rectangular fins can enhance the thermal performance considerably and that an optimum fin interruption exists. A new compact correlation is proposed for calculating the optimum interruption length. The results of this work can be used to improve natural heat transfer from enclosures in a variety of electronics, power electronics and telecom applications.

Wall Temperature Effects on Heat Transfer Coefficient for High-Pressure Turbines

For external convective heat transfer, a common assumption based on the conventional wisdom is that aerodynamics determines the heat transfer coefficient without being affected by wall temperature. In this paper, a systematic computational study has been carried out on the effects of the wall thermal boundary condition on the external heat transfer coefficient for a two-dimensional nozzle guide vane section. Heat transfer coefficients are worked out first in a typical uncoupled manner with specified wall temperatures. As an alternative verification of the findings, further computations are also made using the coupled solid–fluid conjugate heat transfer mode of the solver. The present results show a strong dependence of heat transfer coefficient on wall temperature, in a locally varying manner, which cannot be corrected by using the existing correlations based on a global correction. A detailed examination of the boundary layer highlights the impact of upstream cooling history on the local heat transfer coefficient distribution. Following on these findings, a new three-point nonlinear method to obtain the heat transfer coefficient is proposed. In this method, the heat transfer coefficient has been considered to vary with wall temperature rather than being independent of it as normally assumed. Heat transfer coefficient predictions for different wall temperatures have been compared between the conventional two-point method and the newly proposed one. The effectiveness of the new method has been clearly demonstrated.

Proper evaluation of the external convective heat transfer for the thermal analysis of cool roofs

Cool materials are characterized by high solar reflectance and high thermal emittance; when applied to the external surface of a roof, they make it possible to limit the amount of solar irradiance absorbed by the roof, and to increase the rate of heat flux emitted by irradiation to the environment, especially during nighttime. However, a roof also releases heat by convection on its external surface; this mechanism is not negligible, and an incorrect evaluation of its entity might introduce significant inaccuracy in the assessment of the thermal performance of a cool roof, in terms of surface temperature and rate of heat flux transferred to the indoors. This issue is particularly relevant in numerical simulations, which are essential in the design stage, therefore it deserves adequate attention. In the present paper, a review of the most common algorithms used for the calculation of the convective heat transfer coefficient due to wind on horizontal building surfaces is presented. Then, with reference to a case study in Italy, the simulated results are compared to the outcomes of a measurement campaign. Hence, the most appropriate algorithms for the convective coefficient are identified, and the errors deriving by an incorrect selection of this coefficient are discussed.

Measurements of Adiabatic Film and Overall Cooling Effectiveness on a Turbine Vane Pressure Side With a Trench

Film cooling performance is typically quantified by separating the external convective heat transfer from the other components of the conjugate heat transfer that occurs in turbine airfoils. However, it is also valuable to assess the conjugate heat transfer in terms of the overall cooling effectiveness, which is a parameter of importance to airfoil designers. In the current study, adiabatic film effectiveness and overall cooling effectiveness values were measured for the pressure side of a simplified turbine vane model with three rows of showerhead cooling at the leading edge and one row of body film cooling holes on the pressure side. This was done by utilizing two geometrically identical models made from different materials. Adiabatic film effectiveness was measured using a very low thermal conductivity material, and the overall cooling effectiveness was measured using a material with a higher thermal conductivity selected such that the Biot number of the model matched that of a turbine vane at engine conditions. The theoretical basis for this matched-Biot number modeling technique is discussed in some detail. Additionally, two designs of pressure side body film cooling holes were considered in this study: a standard design of straight, cylindrical holes and an advanced design of “trenched” cooling holes in which the hole exits were situated in a recessed, transverse trench. This study was performed using engine representative flow conditions, including a coolant-to-mainstream density ratio of DR = 1.4 and a mainstream turbulence intensity of Tu = 20%. The results of this study show that adiabatic film and overall cooling effectiveness increase with blowing ratio for the showerhead and pressure side trenched holes. Performance decreases with blowing ratio for the standard holes due to coolant jet separation from the surface. Both body film designs have similar performance at a lower blowing ratio when the standard hole coolant jets remain attached. Far downstream of the cooling holes both designs perform similarly because film effectiveness decays more rapidly for the trenched holes.

The critical heat transfer characteristics of an insulated or compound sphere considering heat radiation

Critical heat transfer from an insulated or compound sphere happen in ambient air with natural convection. Thus, heat radiation in such a situation must be considered because natural convection has only a small effect. It is found that the critical heat transfer characteristics considering heat radiation are very different from those neglecting heat radiation. The conventional critical heat transfer characteristics neglecting heat radiation only depend on sphere size, external convection heat transfer coefficient, and insulation conductivity. In addition to the above parameters, the critical heat transfer characteristics considering heat radiation are also related to the surface emissivities of the sphere and the insulation, the surrounding temperature, and the internal convection heat transfer coefficient.

Aero-thermal investigation of a multi-splitter axial turbine

This paper reports the external convective heat transfer in an innovative low-pressure vane, designed with a multi-splitter configuration. Three aerodynamic airfoils are positioned in between larger structural vanes, replacing struts presently used in current aero-engines, which results in superior aerodynamic performance. Static pressure and heat flux measurements were carried out in the large compression tube facility of the von Karman Institute, using pneumatic taps and single-layered thin film gauges respectively. The steady and unsteady heat transfer distributions were obtained at representative conditions of modern aero-engines, with M 2,is close to unity and a Reynolds number of approximately 10 6. This facility is specially suited to control the gas-to-wall temperature ratio that drives transition mechanisms. The heat transfer across the multi-splittered passages is confronted with correlations on ducts to further characterize the boundary layer status. The data will be used to guide code developers by verifying their boundary layer transition models, and designers by showing the areas of the vane where heat transfer is most sensitive to the off-design conditions.

Mathematical modeling of air duct heater using the finite difference method

Mathematical modeling of air duct heater using the finite difference method In this research, mathematical modeling of a duct heater has been performed using energy conservation law, Stefan-Boltzman law in thermal radiation, Fourier's law in conduction heat transfer, and Newton's law of cooling in convection heat transfer. The duct was divided to some elements with equal length. Each element has been studied separately and air physical properties in each element have been used based on its temperature. The derived equations have been solved using the finite difference method and consequently air temperature, internal and external temperatures of the wall, internal and external convection heat transfer coefficients, and the quantity of heat transferred have been calculated in each element and effects of the variation of heat transfer parameters have been surveyed. The results of modelling presented in this paper can be used for the design and optimization of heat exchangers.

End-wall errors in temperature measurement of external convective heat transfer with Moiré deflectometry

Moiré deflectometry is a robust and simple optical method that allows obtaining the temperature field in flows with uniform pressure and two-dimensional and axisymmetric flows. Since in real configurations it is not possible to keep exactly the hypothesis of two-dimensional flow, it is necessary to asses this influence. Therefore this work studies a procedure to estimate the errors due to the end-wall and 3-D effects when Moiré deflectometry is used for measuring the temperature field of an external convective heat transfer flow with free edges. According to the value of the parameter Gr/Re 2, results show two tendencies. The error in temperature measurement is smaller than 1% for ratios of thermal boundary layer thickness to the test field width lower than 0.4, in turbulent forced convection flows for temperature differences of 40 K. Temperature effects are significative, multiplying the error by two for a temperature difference 50% greater. These results enable researchers to evaluate the errors of this measurement technique associated with the end-wall effect.

Coupled heat and mass transfer in the entrance region of a circular tube with fully-developed parabolic flow and external convective heating

Coupled heat and mass transfer in the entrance region of a circular tube with fully-developed parabolic flow and external heating is analyzed. The closed form solutions of temperature and concentration distributions, local Nusselt numbers based on the total external heat transfer and convective heat transfer inside the tube, as well as the Sherwood number were obtained. The effects of Biot and Lewis number on heat and mass transfer were investigated.