Local Convective Heat Transfer Coefficient Research Articles

CFD simulation of external CHTC on a high-rise building with and without façade appurtenances

Many high-rise buildings have intricate façade appurtenances such as balconies, mullions, or egg-crates shading elements. These façade systems interact with the external microclimate in a complex manner that affects the convective heat transfer coefficient (CHTC) significantly. In this study, CFD and heat transfer simulations are carried out for buildings with balconies, vertical shading element, egg-crates, and a smooth façade. Comparisons of local-CHTC distribution between buildings with and without the façade appurtenances for rooms on different floor heights and located at a corner or a center region of the building are performed. Two equation SST κ-ω turbulence modeling is used for momentum closure. The result of this study shows that the local and surface-averaged CHTC values at the surfaces of each building are dependent on building aerodynamics and forms of the façade appurtenances. For instance, for a 100 m tall building case study with rectangular floor plan, having an egg-crate shading with a depth of 1 m decreased the local-CHTC of a room around the corner-zone by 35%, 37%, and 38% on the 1st, 15th, and 30th floors, respectively compared to a room in building a smooth façade. Thus, necessitating detail local-CHTC variations and wind directionality effect assessments especially on glazed buildings characterized by weak thermal performances.

Measurement of Local Convective Heat Transfer Coefficient of Alumina-Water Nanofluids in a Double Tube Heat Exchanger

Heat transfer coefficient and thermal efficiency of γ-Al2O3/water nanofluids flowing through a double tube heat exchanger were experimentally investigated. The nanoparticles were well dispersed in distilled water at 0.05–0.15 %vol. A large number of experiments were performed at different fluid flow rates under turbulent flow regime (18,000<Re<40,000) and various nanofluid inlet temperatures ranging from 45 °C to 65 °C. The heat transfer coefficients were measured along the length of the heat exchanger. Results showed that the local heat transfer coefficients have an asymptotic behavior. Furthermore, the addition of these small amounts of nanoparticles to the base fluid augmented the heat transfer up to 16% at the best conditions. In the end, the thermal performance factor was calculated to find the optimum condition at which the nanofluid was used. It was shown that the thermal performance factor of this nanofluid could reach to 1.11. This value was obtained at the nanoparticle concentration of 0.15 vol.% and Reynolds number 18000

Experimental study on heat transfer characteristics of annular flow between vertical concentric pipe

Concentric pipe heat exchangers have been utilized extensively in practical project, such as petroleum, chemical, geothermal energy utilization and other fields. The study of annular flow between vertical concentric pipes is dominated by boiling heat transfer, while the study of forced convection heat transfer for single-phase flow is scarce. In this paper, an experimental investigation is carried out to study of the flow and heat transfer characteristics of the annular flow between vertical concentric pipes. The annular in a vertical concentric pipe heat exchanger with 25mm ring gap is tested with different volume flow rates and different constant heat flow densities. The results show that the Nusselt number of annular flow between vertical concentric pipes increases with Reynolds number. The local convective heat transfer coefficients reduce with the distance from the top to the bottom, which lead to a gradually increased trend in the fluid temperature along the flow direction. Based on the experimental data, an empirical equation of the Nusselt number is obtained to evaluate the overall heat transfer performance of vertically concentric pipe heat exchangers.

Investigation of heat transfer characteristics in the developing and the developed flow of nanofluid inside a tube with different entrances in the transition regime

In this paper, we studied heat transfer characteristics in the developing and the developed flow of nanofluid inside a tube with different entrances in the transition regime. A range of Reynolds numbers from Re = 500 to 13,000 was stimulated to examine flow characteristics in the tube. Effect of entrance, diameter of nanoparticles, nanoparticle type, Reynolds number and concentration of nanoparticles on onset of transition were studied. The results show that heat transfer coefficient and Nusselt number of base fluid and the nanofluid increase with increasing Reynolds number. Also, convective heat transfer coefficient of nanofluid increases by increasing Reynolds number, and the rate of increase in heat transfer coefficient varies depending on diameter, volume fraction and type of nanoparticles. Furthermore, the results show that local Nusselt number and local convective heat transfer coefficient slightly decrease with increasing particle diameter at constant volume fraction of nanoparticles.

Downstream radiative and convective heating from methane and propane fires with cross wind

Experiments were conducted to elucidate the radiative and convective heating occurring downstream of wind-driven fires produced by a gaseous burner. These flames model, at reduced scale, some of the dynamics observed in wind-driven fire spread through wildlands, buildings, mines or tunnels. Methane and propane were used to create fires ranging from 5 to 25 kW with ambient velocities ranging from 0.6 to 2.2 m/s. The total and incident radiative heat flux to a nearly-adiabatic downstream surface were measured by a water-cooled total heat flux gauge and a radiometer, respectively. The interaction between the buoyancy induced by the flame and momentum from the free stream was represented by a mixed-convection parameter, ξ=Grx2/Rex1n, where n = 3/2, 2 or 5/2. ξ was evaluated with two length scales in order to capture effects of both the boundary layer development length (x1) and heated distance downstream of the burner (x2). Results showed that the propane flame (high luminosity) exhibited slightly higher radiative heat fluxes than methane flames (low luminosity) under the same external conditions, while the convective heat flux followed an opposite trend. The downstream local radiative heat flux was quantified using a dimensionless flame thickness δx*, which showed a good relationship with ξ for n = 5/2 but not 3/2 or 2. The local convective heat transfer coefficient was expressed in the form of a local Nusselt number, Nux2Rex1−1/2, and correlated well as a piecewise function with ξ for n = 5/2. It was found that both δx* and Nux2Rex1−1/2 have a turning point at ξ ≈ 0.005, which was visually shown to denote the location where transition between an attachment and plume-like flame occurs. By separately describing both radiative and convective downstream heating, the mechanisms controlling heating which drives flame spread in wind-driven fires can be further understood.

Experimental and numerical studies on convective heat transfer of supercritical R-134a in a horizontal tube

Experimental studies on convective heat transfer characteristics of supercritical R-134a flowing in a horizontal tube with an inner diameter 8 mm were conducted in the range of mass flux 500–1000 kg m−2 s−1, heat flux 20–100 kW m−2 s−1 and pressure 4.3–4.8 MPa. Local wall temperatures and convective heat transfer coefficients of the top and bottom surfaces of the test tube were obtained, and their variational trends with increasing heat flux and mass flux were discussed within the experimental range. To have a quantitative and more comprehensive analysis of convective heat transfer behaviors of supercritical fluids as compared with the conventional methods, two new field synergy analytical methods designated as TCEH and FCEH respectively were developed, motivated by the two basic concepts of the field synergy principle respectively, i.e., the analogy of convective heat transfer to conductive heat transfer and the three general criteria associated with convective heat transfer enhancements. Numerical simulations were performed within the identical parameter ranges, and the effects of buoyancy, heat flux and mass flux on convective heat transfer were analyzed using these two new methods. Finally some new insights into supercritical convective heat transfer were obtained.

Numerical Study on Thermal Hydraulic Performance of Supercritical LNG in Zigzag-Type Channel PCHEs

In this paper, we study a promising plate-type heat exchanger, the printed circuit heat exchanger (PCHE), which has high compactness and is suitable for high-pressure conditions as a vaporizer during vaporization. The thermal hydraulic performance of supercritical produce liquefied natural gas (LNG) in the zigzag channel of PCHE is numerically investigated using the SST κ-ω turbulence model. The thermo-physical properties of supercritical LNG from 6.5 MPa to 10MPa were calculated using piecewise-polynomial approximations of the temperature. The effect of the channel bend angle, mass flux and inlet pressure on local convection heat transfer coefficient, and pressure drop are discussed. The heat transfer and pressure loss performance are evaluated using the Nusselt and Euler numbers. Nu/Eu is proposed to evaluate the comprehensive heat transfer performance of PCHE by considering the heat transfer and pressure drop characteristics to find better bend angle and operating conditions. The supercritical LNG has a better heat transfer performance when bend angle is less than 15° with the mass flux ranging from 207.2 kg/(m2·s) to 621.6 kg/(m2·s), which improves at bend angle of 10° and lower compared to 15° at mass flux above 414.4 kg/(m2·s). The heat transfer performance is better at larger mass flux and lower operating pressures.

An experimental study on flow and heat transfer characteristics of binary hydrate slurries in a horizontal tube

Fluid flow and heat transfer characteristics of two kinds of binary hydrate slurries CO2-TBAB and CO2-THF in a horizontal tube are investigated experimentally. It is found that CO2-TBAB hydrate slurry shows Herschel-Bulkley (H–B) fluid characteristics at low flow velocity (<0.35 m/s) and as a dilatants fluid at high flow velocity (>0.45 m/s). On the other hand, CO2-THF hydrate slurry exhibits dilatant fluid properties in the flow velocity range of 0.1–0.8 m/s. Moreover, heat transfer characteristics of CO2-TBAB slurry are studied at different TBAB initial concentrations, flow velocities and heating powers. The local convective heat transfer coefficient decreases in the slurry region and then increases in the aqueous solution region. The hydrate slurry produced with 15 wt% initial concentration of TBAB shows a longer slurry region and a shorter solution region. The local convective heat transfer coefficient of CO2-TBAB hydrate slurry with 5 wt% TBAB is in a range of 4413–5397 W/m2K and its phase change zone is at a temperature of 12–14 °C.

Heat Transfer and Friction Measurement in Pin-Fin Arrays Under Mist Flow Condition

Abstract Effects of air/water mist flow on endwall heat transfer in a square channel were experimentally investigated using infrared thermography. The purpose was to study the detailed heat transfer contour variation caused by the generation of the liquid films. The surface was roughened with staggered partial pin-fin arrays to enhance flow mixing and liquid entrainment. Two streamwise spacings (Xp/d = 3 and 6) of the fin array were investigated. The gas Reynolds number ranged from 7900 to 24,000. The calculated droplet deposition velocity was comparable to the literature results and was not substantially affected by the gas Reynolds number or fin spacing. For the pin-fin array, heat transfer was dominated by the water accumulation and liquid film formation, which was dependent on the carrier gas flow rate and fin spacing. Furthermore, thick liquid fragments entrained between the fins substantially enhanced local convective heat transfer coefficients. The average heat transfer enhancement on the finned surfaces using mist flow was four times as high as the air flow. The pressure drop from the mist flow was two times as high as the air flow.

Modelling Cooling of Packaged Fruit Using 3D Shape Models

This study presents a novel methodology to model the cooling processes of horticultural produce using realistic product shapes rather than commonly used simplified 3D shapes, such as spheres. Variable 3D apple and pear models were created by means of a validated geometric model generator based on X-ray computed tomography images. The fruit were randomly stacked into a geometrical model of a corrugated fibreboard box using the Discrete Element Method. A forced-air cooling process was simulated for three such apple filling patterns using CFD and the results were compared to those obtained with fruit represented by equivalent spheres. No significant difference in average aerodynamic resistance between the real apple shape and its spherical representation was found. The main contributor to the overall pressure drop was the package design rather than product shape. However, large differences in local air velocity and convective heat transfer coefficients were found between the two representations. The degree of cooling uniformity between individual fruit was overestimated when using simplified product shapes: real apple fruit shapes cooled less uniform. This difference between real and simplified product shapes was even larger for a box filled with pear fruit that are more different from a spherical shape. These results demonstrate that improved computer-aided design approaches help in simulating more accurate convective cooling processes. In a next step, such simulations will be used for multi-objective optimization of packaging in terms of cold chain efficiency and cooling uniformity.

Heat Transfer Augmenation Using Nanofluid of a Confined Slot Impingement Jet on Concave Surface

Numerical investigation of heat transfer process using A12O3-water nanofluid in a confined slot jet impingement on concave surface is performed. The simulation is conducted on a two-dimensional turbulent flow with k –ω SST model for the turbulence computation. A constant heat flux is considered on concave surface. Different parameters such as various Reynolds numbers, and nozzle-to-surface spacing have been considered to study the flow field and convective heat transfer performance of the system. Results in form of distribution of local and average Nusselt number and Convective heat transfer coefficients at the curved surface are shown to elucidate the heat transfer and fluid flow process. In addition, qualitative analysis of both stream function and isotherm contours is carried out to perceive the flow pattern and heat transfer mechanism due to addition of nanofluids. The results reveal that average Nusselt number as well as heat transfer coefficient significantly rises with jet inlet Reynolds number. It is also proved that heat transfer is augmented when nanoparticles are added to a base fluid.

A non-intrusive technique to determine the spatially varying heat transfer coefficients in a flat plate with flush mounted heat sources

In this work, a novel experimental technique is developed to estimate spatially varying heat transfer coefficients from a flat plate with flush mounted discrete heat sources, using Bayesian inference with temperature measurements from liquid crystal thermography (LCT) at an adiabatic surface of the plate without disturbing the fluid flow. Steady state, laminar forced convection experiments have been done on a flat Bakelite plate with three identical embedded discrete aluminium heat sources of dimensions 0.16 × 0.06 × 0.015 (l × w × t all in m). The variation of local convective heat transfer coefficient is obtained in the form of a Nusselt number correlation Nu=aReb(x/l)c. This correlation is first developed by limited numerical simulations for two dimensional conjugate convection. With this correlation, a computationally less complex problem of conjugate conduction in the flat plate also known as the forward model is repeatedly solved for various values of 'a', 'b' and 'c' to obtain the temperature distributions at select points on the adiabatic surface using COMSOL. A surrogate model obtained by Artificial Neural Networks (ANN) built upon the data from these simulations then replaces the forward model. This surrogate model is used to drive a Markov Chain Monte Carlo based Metropolis Hastings algorithm to generate the samples to the forward model to solve the inverse problem of getting 'a', 'b' and 'c' from temperature measurements at the adiabatic surface. Bayesian framework is then adopted to compare the experimental and the simulated temperatures to generate posteriors and the mean, maximum a posteriori and standard deviation of the parameters 'a', 'b' and 'c' are estimated. The effect of number of samples and the temperature points on the performance of the estimation process has been reported. Finally, with the retrieved values of 'a', 'b' and 'c' temperature distributions are obtained by solving the conduction problem and these are compared with those actually measured with TLC.

Convective Heat Transfer Coefficients of Multifaceted Longitudinal and Transversal Bricks of Lattice Setting in Tunnel Kilns

This paper reports the local multifaceted and area-averaged convective heat transfer coefficients (CHTCs) of longitudinal and transverse bricks arranged in lattice brick setting in tunnel kilns, using a three-dimensional (3D) computational fluid dynamics (CFD) model. A mesh sensitivity analysis was performed and the model was validated against reported experimental data in tunnel kilns. Three turbulence models were tested: the standard k–ε, re-normalization group (RNG) k–ε, and k–ω. The k–ω model provided the closest results to the experimental data. The CHTCs from the front, back, left, and right faces of the longitudinal and transverse bricks were calculated under various conditions. Area-averaged CHTCs for bricks were determined from the multifaceted CHTCs. Effects of rows, layers, and walls on faces and area-averaged CHTCs were investigated. A sensitivity analysis was performed to explore the effect of flow channels on the CHTCs. The numerical results showed that the CHTCs are enhanced by 17% for the longitudinal bricks and 27% for the transverse bricks when a uniform flow is reached in the tunnel kilns. Also, similar area-averaged CHTCs for the longitudinal and transverse bricks were obtained as a result of the uniform flow. Therefore, the specific energy consumption, quality, and quantity of brick production could be enhanced.

Numerical flow simulation of the neutron source SINQ of PSI

The Swiss spallation source, SINQ, is in operation since 1996. Since its start-up a constant target development program has been pursued, which culminated in the operation of the liquid metal target MEGAPIE. However, the work-horse target consists of a bundle of Zircaloy-II rods filled with Lead, the so-called Cannelloni target. As the high intensity proton accelerator, HIPA, at the PSI constantly delivers higher beam currents to SINQ it is important to perform numerical simulations to understand the response of the target to the power deposition and to ensure the proper D2O cooling of the Cannelloni rod bundle. Steady-state 3D fluid simulations of the heavy water coolant flow are coupled with the heat source terms in the Cannellonis. The source terms are described as UDFs (User Defined Function) for all Cannellonis with its Zircaloy-II tube and Lead filling. The energy deposition functions for the heat source terms are calculated using MPCNX 2.7.0. Major results from the CFD simulations are the flow structure with the resulting pressure field and the temperature distribution in the target. The results show good agreement with respect to the measured values during operation of SINQ. Additionally the local convective heat transfer coefficients (HTC) are calculated.

The Challenge to Measure Single-phase Convective Heat Transfer Coefficients in Microchannels

ABSTRACTThe determination of local convective heat transfer coefficients in microfluidics is a very hard task. Due to the small dimensions of channels and walls, the use of conventional measurement techniques is only partially suitable in microfluidics. For this reason, a strong effort has been made during the last decades in order to propose innovative techniques which use internal (to microdevices) sensors of reduced dimensions and/or external conventional sensors. In this paper a review of the main experimental techniques proposed for the determination of the local near-wall fluid temperature, the local wall temperature, and the local fluid bulk temperature will be given by putting in evidence for each technique's positive and negative aspects as well as their actual limitations with the aim to stimulate and address the research on this topic in the near future. The problems and the limitations existing nowadays for the accurate measurements of the local thermal properties of a convective microflow demonstrate that for the analysis of microconvection experimental data have to be always integrated by a numerical modeling of the observed system.

Numerical analysis on enhanced performance of new coaxial cross twisted tapes for laminar convective heat transfer

Owing to the high viscosity of lubricating oil, oil coolers meet the disadvantages of low heat transfer coefficients and thus large volumes. In order to enhance the heat transfer performance, a new type of twisted tape, called coaxial cross twisted tape, is proposed based on the field synergy theory and traditional twisted tapes. The numerical analysis of the heat transfer performance is presented in the article using CFD software STAR-CCM+. The local flow characteristics, and local and average convective heat transfer coefficients are analyzed, when the Reynolds number ranges from 40 to 1050. The effects of the four different twist ratios of 2.0, 3.0, 4.0 and ∞ on the heat transfer performance are also investigated. To evaluate the effect of heat transfer enhancement under given pumping power, the performance evaluation criterion (PEC) is discussed. The results indicate that the local convective heat transfer coefficient increases markedly with the insertion of the new twisted tapes because of the increase of the velocity near wall and thickness reduction of thermal boundary layer. The more flow zones are separated by the coaxial cross twisted tapes, the larger is the temperature gradient near wall and the higher is convective heat transfer coefficient. The Nusselt number and PEC of the heat transfer tube fitted with the new coaxial cross twisted tapes have increased by 151–195% and 90–123% of the traditional twisted tape, respectively.

Experimental study of natural convection heat transfer from a nonuniformly heated flat plate simulating PV panel

In view of the characteristic of Photovoltaic (PV) conversion, an experimental study has been conducted to investigate the natural convection heat transfer from a flat plate. In order to simulate the real PV cells, three electrical heating circuits were employed to achieve a linear nonuniform heat flux boundary condition. The major parameters, such as the gradient parameter of heat flux k, tilt angle θ and heat flux qw were introduced to analyze their influence on heat transfer ability. Both the local temperature distribution and the overall trend of Nusselt number have been presented. Comparing with the uniform thermal boundary condition, the results show that the local convection heat transfer coefficient is fluctuated at the positions where heat flux has changed. When gradient parameter of heat flux increases, the difference of local convection heat transfer coefficient at the top of the plate between tilt angle θ = 0° and 90° becomes smaller. Moreover, it is observed that the gradient parameter of heat flux has a promotion effect on average convection Nusselt number at larger tilt angle, but the effect becomes complex as tilt angle is smaller. Finally, a correlation combining all significant factors has been put forward to estimate the natural convection heat transfer.

A numerical model for boiling heat transfer coefficient of zeotropic mixtures

Zeotropic mixtures never have the same liquid and vapor composition in the liquid-vapor equilibrium. Also, the bubble and the dew point are separated; this gap is called glide temperature (Tglide). Those characteristics have made these mixtures suitable for cryogenics Joule-Thomson (JT) refrigeration cycles. Zeotropic mixtures as working fluid in JT cycles improve their performance in an order of magnitude. Optimization of JT cycles have earned substantial importance for cryogenics applications (e.g, gas liquefaction, cryosurgery probes, cooling of infrared sensors, cryopreservation, and biomedical samples). Heat exchangers design on those cycles is a critical point; consequently, heat transfer coefficient and pressure drop of two-phase zeotropic mixtures are relevant. In this work, it will be applied a methodology in order to calculate the local convective heat transfer coefficients based on the law of the wall approach for turbulent flows. The flow and heat transfer characteristics of zeotropic mixtures in a heated horizontal tube are investigated numerically. The temperature profile and heat transfer coefficient for zeotropic mixtures of different bulk compositions are analysed. The numerical model has been developed and locally applied in a fully developed, constant temperature wall, and two-phase annular flow in a duct. Numerical results have been obtained using this model taking into account continuity, momentum, and energy equations. Local heat transfer coefficient results are compared with available experimental data published by Barraza et al. (2016), and they have shown good agreement.

Experimental study on flow and heat transfer of power law fluid in structured packed porous media of particles

Fluid flow and heat transfer characteristics of power law fluid in wall bounded three-dimensional structured packed beds of spheres was investigated experimentally. The packed beds of spheres were in Simple Cubic (SC) configuration and enclosed in square cross-section duct. The Partially Hydrolyzed Polyacrylamide (HPAM) solutions with different concentrations and temperatures were chosen as the working fluids, of which the rheological behavior can be described by a power-law type non-Newtonian fluid.The relationship between the pressure-drop and flow velocity was verified by the modified Ergun type equations. The flow regime boundaries, the coefficients AE′, BE′, and the critical particle Reynolds number in the equations were discussed for the various concentration and temperature. And Variations of the local (or mean) convection heat transfer coefficient of HPAM solutions with the power index, Reynolds number and heat flux were examined and discussed. Also the concept of thermal efficiency was introduced to gain the comprehensive evaluation of its flow and heat transfer characteristics. At last, the temperature difference between particle and fluid along the flow direction was analyzed. The results can provide a theoretical basis and enrich the basic experiment data for the flow and heat transfer mechanism of the power-law type non-Newtonian fluid flowing through the three-dimensional structured packed porous media of particle.

Pore-scale numerical simulation of fully coupled heat transfer process in porous volumetric solar receiver

A fully coupled heat transfer model at pore-scale of volumetric solar receiver is established in this paper. The X-ray computed tomography technique is applied to reconstruct the porous structure. By generating the voxel mesh and coupling with the Monte Carlo Ray Tracing method, the energy source due to the solar radiation could be determined and then added to the unstructured CFD mesh. The governing equations are solved by the commercial CFD software FLUENT. The results show that the details of the fluid flow and heat transfer in volumetric solar receiver are successfully captured. The pressure drop correlation corresponds satisfactorily to the previous study. The local convective heat transfer coefficient varies in a small range along the inlet fluid flow direction inside volumetric solar receiver and the average Nusselt number could be correlated to a power function of the Reynolds number. The radiation transfer inside the porous media is visualized and thermal radiation loss is evident at the entrance of the solar receiver. The proportion taken by radiation in the total heat transfer is determined as a function of the average temperature of porous skeleton.