Experimental study on laminar flow forced-convection in a channel with upper V-corrugated plate heated by radiation

Experimental study of the effects of the operating parameters on laminar flow forced-convection heat transfer for air flowing in a channel having a V-corrugated upper plate heated by radiation heat flux while the other walls are thermally insulated has been carried out. The parameters studied and their ranges were as follows: flow Reynolds number (Re) ranging from 750 to 2050, incident radiation fluxes (q_inc) of 400, 700, and 1000 W/m², inlet air bulk temperatures (T_b,in) ranging from 12.4 to 59.4 °C and tilting angles of the channel (β) of 0°, 15°, 30°, 45°, and 60°. The results show that, the effect of Re on local Nusselt number (Nu_x) are clear and more significant at the channel entrance region. While, changing β from 0° to 60° leads to an increase in Nu_x by a ratio ranging from 33% to 67.3% depending on Re values and other operating parameters. Increasing the q_inc values by 175% and 250% leads to an increase in Nu_x values by 26% and 50%, respectively. In addition, the results indicate that there are significant increases in Nu_x in the channel entrance region due to the increase in inlet air bulk temperature and this influence diminishes downstream.     

Novel thermoelectric generator heat exchanger for indirect heat recovery from molten CuCl in the thermochemical Cu–Cl cycle of hydrogen production

The looming threat of global warming has elicited efforts to develop reliable sustainable energy resources. Hydrogen as a clean fuel is deemed a potential solution to the problem of storage of power from renewable energy technologies. Among current thermochemical hydrogen generation methods, the thermochemical copper-chlorine (Cu–Cl) cycle is of high interest owing to lower temperature requirements. Present study investigates a novel heat exchanger comprising a thermoelectric generator (TEG) to recover heat from high temperature molten CuCl exiting the thermolysis reactor. Employing casting/extrusion method, the performance of the proposed heat exchanger is numerically examined using COMSOL Multiphysics. Results indicate that maximum generated power could exceed 40 W at the matching current of 4.5 A. Maximum energy conversion efficiency yields to 7.1%. Results demonstrate that TEG performance boosts with increasing the inlet Re number, particularly at the hot end. For the molten CuCl chamber, findings denote that there is a 36% discrepancy between highest and lowest Re numbers. Similarly, the highest efficiency value pertains to the case with the highest inlet velocity. Moreover, the highest temperature difference between inlet and outlet of the cooling water is about 28 °C and 10 °C for the lowest and highest inlet Re numbers, respectively. Average deviation from anticipated friction factor and Nusselt number are 0.31% and 12.62%, respectively.     

Numerical study of local heat transfer coefficient and fin efficiency of wavy fin-and-tube heat exchangers

Three-dimensional numerical simulations were performed for laminar flow of wavy fin-and-tube heat exchangers by using body-fitted coordinates (BFC) method with fin efficiency effect accounted. The prediction results of average Nusselt number, friction factor and fin efficiency were compared with the related experimental correlations [R.C. Xin, H.Z. Li, H.J. Kang, W. Li, W.Q. Tao, An experimental investigation on heat transfer and pressure drop characteristics of triangular wavy fin-and-tube heat exchanger surfaces, J. Xi'an Jiaotong Univ. 28 (2) (1994) 77–83] and Schmidt approximation [T.E. Schmidt, Heat transfer calculations for extended surfaces, Refrigerating Engineering (April 1949) 351–357]. For Reynolds numbers based on the tube outside diameter ranging from 500 to 4000, the mean deviation is 3.3% for Nusselt number, 1.9% for friction factor and 3.6% for fin efficiency. The distributions of local Nusselt number and fin efficiency on fin surface were studied at wavy angle equal to 0° (plain plate fin), 10° and 20° respectively. The local Nusselt number decreases along the air flow direction, but fin efficiency increases in general. The wavy angle can greatly affect the distributions of local Nusselt number and fin efficiency, and make the distributions present fluctuation along the flow direction. The result also shows that the fin efficiency at the inlet region of wavy fin is larger than that of plain plate fin at the same region. With the increase of Reynolds number, the effects of wavy angle on the distributions of local Nusselt number and fin efficiency are more and more significant.     

Experimental study on heat transfer augmentation in a double pipe heat exchanger utilizing jet vortex flow

This paper examines experimentally the effect of jet vortex technology on enhancing the heat transfer rate within a double pipe heat exchanger by supplying the heat exchanger with water at different vortex strengths. A vortex generator with special inclined holes with different inlet angles was designed, manufactured, and integrated within the heat exchanger. In this study, four levels of Reynolds number for hot water in the annulus (Re_h) were used, namely, 10,000; 14,500; 18,030; and 19,600. Similarly, four levels of Reynolds number for cold water in the inner tube (Re_c) were used, namely, 12,000; 17,500; 22,500; and 29,000. As for the inlet flow angle (θ), four different levels were selected, namely, 0°, 30°, 45°, and 60°. The temperature along the heat exchanger was measured utilizing 34 thermocouples installed along the heat exchanger. It was found that increasing the inlet flow angle (θ) and/or the Reynolds number results in an increase in the local Nusselt number, the overall heat transfer coefficient, and the ratio of friction factor. It is revealed that the percentage increase in the average Nusselt number due to swirl flow compared to axial flow was 10%, 40%, and 82% for an inlet flow angle of 30°, 45°, and 60°, respectively.     

Investigation of heat transfer and pressure drop in a concentric heat exchanger with snail entrance

In this study, in order to increase heat transfer in concentric double-pipe heat exchangers by passive method, snail which is mounted at inlet of the inner pipe and assumed as a swirl generator was used. In the experimental set-up, cold air in ambient conditions was passed through the inner pipe while hot water was flowing through the annulus. The effects of a snail on the heat transfer and pressure drop were investigated for parallel and counter-flow, and obtained Nusselt numbers (Nu) were compared with those found, using a standard correlation such as Dittus–Boelter equation given for axial flows in smooth pipes. The results were correlated in the form of Nusselt number as a function of Reynolds number, Prandtl number and the swirling angle. An augmentation of up to 120% in Nusselt number was obtained in the swirl flow for counter-flow and 45° swirling angle. Though the swirl flow effect of the snail caused some increase in pressure drop, this effect was unimportant compared with the improvement in heat transfer capacity.     

An experimental investigation of heat transfer characteristics for steam cooling in a rectangular channel with parallel ribs

An experimental study of heat transfer characteristics in superheated steam cooled rectangular channels with parallel ribs was conducted.The distribution of the heat transfer coefficient on the rib-roughed channel was measured by IR camera.The blockage ratio(e/Dh) of the tested channel is 0.078 and the aspect ratio(W/H) is fixed at3.0.Influences of the rib pitch-to-height ratio(P/e) and the rib angle on heat transfer for steam cooling were investigated.In this paper,the Reynolds number(Re) for steam ranges from 3070 to 14800,the rib pitch-to-height ratios were 8,10 and 12,and rib angles were 90°,75°,60°,and 45°.Based on results above,we have concluded that:In case of channels with 90° tranverse ribs,for larger rib pitch models(the rib pitch-to-height ratio=10 and12),areas with low heat transfer coefficient in front of rib is larger and its minimum is lower,while the position of the region with high heat transfer coefficient nearly remains the same,but its maximun of heat transfer coefficient becomes higher.In case of channels with inclined ribs,heat transfer coefficients on the surface decrease along the direction of each rib and show an apparent nonuniformity,consequently the regions with low Nusselt number values closely following each rib expand along the aforementioned direction and that of relative high Nusselt number values vary inversely.For a square channel with 90° ribs at Re= 14800,wider spacing rib configurations(the rib pitch-to-height ratio=10 and 12) give an area-averaged heat transfer on the rib-roughened surface about8.4%and 11.4%more than P/e=8 model,respectively;for inclined parallel ribs with different rib angles at Re=14800,the area-averaged heat transfer coefficients of 75°,60° and 45° ribbed surfaces increase by 20.1%,42.0%and 44.4%in comparison with 90° rib angle model.45° angle rib-roughened channel leads to a maximal augmentation of the area-averaged heat transfer coefficient in all research objects in this paper.     

Experimental study of thermal performance improvement in a cross-flow heat exchanger by using copper foam

In this study, copper foam was used as a porous medium in place of traditional aluminum fins. A comparison between the two heat exchangers—one with fins and the other with copper foam—was conducted under various conditions. The air inlet velocity ranged from 0.9 to 9.3 m/s, and the water inlet temperature ranged from 10°C to 18°C. Different water flow rates were tested. A comparison was made between the performance of copper foam and aluminum fins by calculating several parameters, including thermal resistance, heat exchanger effectiveness, Colburn factor, Nusselt number, friction factor, and area goodness factor (AG). The experimental results showed that at low air velocities, the heat transfer coefficient for both types of heat exchangers was almost equal. However, at high air velocities, the copper foam exhibited a higher heat transfer coefficient. The Colburn factor was higher for the heat exchanger with copper foam than in the conventional fins, where it was equal to 0.1959 for the copper foam and 0.1186 for the fins. On the other hand, the AG was higher in the case of fins than in the heat exchanger with copper foam.     

A new method for heat transfer and fluid flow performance simulation of plate heat exchangers

Successful numerical simulation on heat transfer and fluid flow performances of plate heat exchangers is vital. Their complex structures often make the numerical calculation quite difficult and time-consuming. Conclusions drawn by the present work are promising for greatly simplifying the simulation. Different types of plates consisting of different numbers of periods are analyzed and it is concluded that the Nusselt number remains constant for different periods of different plates under different inlet velocities. The central friction coefficients behave the same as Nusselt number. For the first and last periods, the respective friction coefficient also remains for different plates. A small plate fraction with four periods is enough for performance prediction of any-sized plates.     

Forced heat convection heat transfer from shrouded staggered fin arrays

The heat transfer characteristics of staggered, frontally shrouded fin arrays have been studied experimentally, in the case of forced convection of air. Nusselt number and Colburn number are given, for the range of Reynolds numbers from 2000 to 12000. The local Nusselt number distributions signal the existence of the periodically developed temperature field. Average Colburn number values are found to be in agreement, from an engineering standpoint, with correlations predicted for offset strip-fin heat exchangers.     

Experimental Investigation of Heat Transfer and Resistance Characteristics of a Finned Oval-Tube Heat Exchanger With Different Air Inlet Angles

The air inlet flow direction is not orthogonal to the heat exchanger surface in many cases. To study the performance of the heat transfer and pressure drop of a heat exchanger with different air inlet angles, this paper shows the experimental system about a finned oval-tube heat exchanger inclined toward the air incoming flow direction. The heat transfer and pressure drop characteristics of four air inlet angles (90°, 60°, 45°, and 30°) are studied separately for the Reynolds number ranging from 1300 to 13000 in this study. The experimental correlations of Nusselt number and resistance coefficient of the air side are acquired. The results show that the overall heat transfer coefficients become smaller and smaller with the decrease of the air inlet angles, while the pressure drops have significant changes. The heat transfer performances of the heat exchanger under the three inclined air inlet angles are worse than that at 90°. Among the three inclined angles, the performance at 45° is the best under identical mass flow rate criterion and at low Reynolds number under identical pressure drop criterion; that at 60° is the best at large Reynolds under identical pressure drop criterion. Finally, some conclusions are attained about the effects of the air inlet angles on the heat transfer and pressure drop performance of the finned oval-tube heat exchanger.     

Prediction of local heat transfer characteristics of dilute gas‐solid flows through an adiabatic,horizontal pipe

The local heat transfer characteristics of gas‐solid flows through an adiabatic, horizontal pipe are numerically studied using the two‐fluid model of Ansys Fluent 15. First, the model is validated with the experimental results available in the literature for the air temperature and average Nusselt number. Then, the local heat transfer characteristics of gas‐solid flows, such as temperature profiles of gas and solid, gas‐solid Nusselt number, logarithmic mean temperature difference, and effectiveness of gas and solid, are studied by changing different parameters (gas velocities 15‐24 m/s; inlet solid loading ratios 0.1‐1; particle diameters 100‐400 µm). It is observed that increasing the particle diameter and inlet gas velocity increases the gas temperature and decreases the solid temperature, increases the logarithmic mean temperature difference, and decreases the thermal effectiveness of gas and solid. However, increasing the solid loading ratio decreases the gas and solid temperatures, decreases the logarithmic mean temperature difference, and increases the thermal effectiveness of gas and decreases the thermal effectiveness of solid. Moreover, increasing the particle diameter decreases the gas‐solid Nusselt number, whereas increasing the solid loading ratio and inlet gas velocity increase the gas‐solid Nusselt number.     

Analytical solution to the problem of heat transfer in an MHD flow inside a channel with prescribed sinusoidal wall heat flux

An MHD laminar flow through a two dimensional channel subjected to a uniform magnetic field and heated at the walls of the conduit over the whole length with a sinusoidal heat flux of vanishing mean value or not, is studied analytically. General expressions of the temperature distribution and of the local and mean Nusselt numbers are obtained by using the technique of linear operators in the case of negligible Joule and viscous dissipation and by taking into account the axial conduction effect. The principal results show that an increase of the local Nusselt number with Hartmann number is observed, and, far from the inlet section, the average heat transfer between the fluid and the walls shows a significant improvement at all values of Hartmann number used when the frequency of the prescribed sinusoidal wall heat flux is increasing in the case of vanishing mean value of the heat flux and this is true especially at low Peclet numbers.     

Effect of surfactant addition on boiling heat transfer in a liquid film flowing in a diverging open channel

An experimental study was conducted to investigate how the addition of small amounts of a surfactant influences the heat transfer characteristics in a thin boiling liquid film flowing in a diverging open channel. Heat transfer experiments were conducted with fluid inlet temperatures from 40 °C to 92 °C. The flow field on the plate included thin film supercritical flow upstream of a hydraulic jump and thick film subcritical flow downstream of a hydraulic jump. Nusselt numbers for the non-boiling heat transfer without surfactant addition scaled linearly with the film Reynolds number. The boiling heat transfer produced higher Nusselt numbers with a weaker dependence on the Reynolds number. Experimental results showed that a boiling surfactant solution created a thick foam layer with high heat transfer rates and Nusselt numbers that are very weakly dependent on the inlet flow rate or the inlet Reynolds number.     

Combined convection and radiation in participating laminar flow inside a parallel-plate duct with flux boundary conditions

The interaction of radiation and forced convection in hydrodynamically developed, thermally developing steady laminar forced flow of a gray fluid between two infinite parallel plates with the prescribed heat flux boundary conditions is investigated. The implicit finite difference scheme is applied to solve the energy equation, while high order P_N method is used to solve the radiation part of this problem. The effects of the conduction-to-radiation parameter, the single scattering albedo, the optical thickness, the surface reflectivity of the plates and the inlet temperature on the surface temperature and local Nusselt number are studied.     

螺旋管内水和蒸汽局部传热特性研究

以高温气冷堆蒸发器为背景,采用FLUENT软件模拟了单相水和蒸汽在不同尺寸螺旋管内部的流动和传热过程,研究了壁面局部传热特性。计算结果表明,远离螺旋中心线一侧局部传热较强而靠近螺旋中心线一侧传热较弱,壁面Nu周向分布非常不均匀。管径与螺旋直径之比是主要影响因素,当其值增大时截面温度极值点向螺旋中心线外侧移动,加剧了温度分布和Nu分布的不均匀性。在层流向湍流过渡区内,Re的增大使截面各点温度梯度均有所增加,同时也增大了Nu周向分布的不均匀程度,但在旺盛湍流区内Re对Nu分布无明显影响。壁面热边界条件形式对局部Nu周向分布没有显著影响。给出了局部Nu的估算式。     

COMBINED RADIATION AND LAMINAR MIXED CONVECTION IN THE THERMAL ENTRANCE REGION OF HORIZONTAL ISOTHERMAL RECTANGULAR CHANNELS

The interaction of thermal radiation with laminar mixed convection for a gray fluid in the thermal entrance region of a horizontal isothermally heated rectangular channel is numerically investigated. The vorticity-velocity formulation of the Navier-Stokes equation and the integral formulation for radiation solved by finite-element nodal approximation are employed. The effects of radiation and convection on local Nusselt number, the development of bulk temperature, and the friction factor are examined. Secondary flow induced by the buoyancy effects leads to a significant enhancement in heat transfer in the entrance region. The result shows that the existence of secondary flow causes fluctuations in local Nusselt number and this phenomenon is reduced by the effect of thermal radiation and a large aspect ratio.     

Development of new correlations for forced convection heat transfer during cooling of products

This paper presents the new, simple but powerful effective Nusselt–Reynolds correlations for estimating the effective convective heat transfer coefficients of spherical and cylindrical products cooled in water and air flows. In this respect, both experimental and theoretical works were obtained. In the experimental case, several spherical and cylindrical products, namely, tomatoes, pears and cucumbers were cooled in water and air flow and their centre temperature variations were measured. In the theoretical case, the effective convective heat transfer coefficients for the individual spherical and cylindrical products were determined using the centre temperature data in the present approach including Dincer's models. Therefore, the new Nusselt–Reynolds correlations were developed using the effective convective heat transfer coefficient values and a general diagram of Nu/Pr^1/3 against Reynolds number was drawn. This study indicates that the present effective Nu–Re correlations are capable of estimating the effective convective heat transfer coefficients of any spherical and cylindrical shaped products exposed to water and air cooling in practical applications in a simple and accurate manner.     

Laminar heat transfer and friction characteristics of microencapsulated phase change material slurry in a circular tube with twisted tape inserts

It is novel and better method that microencapsulated phase change material (MPCM) slurry and the tube with twisted tape inserts are adopted together to enhance convective heat transfer. In this paper, numerical analyses were carried out to study laminar heat transfer and friction characteristics of MPCM slurry in a circular tube with twisted tape inserts. It is found that the MPCM slurry in the tube with twisted tape insert leads to the best performance of convective heat transfer for the bigger apparent specific heat and the intensive swirl flow. Furthermore, the modified average Nusselt number increases with decreasing bulk Stefan number, twisted ratio and increasing Re, while the friction factor increases with increasing Re and decreasing twisted ratio. It is also found that the heat transfer enhanced efficiency increases with increasing Re and decreasing Ste_b, and the heat transfer enhancement effects of twisted tape for low Ste_b slurry are better than that for high Ste_b slurry. Moreover, the thermal-hydraulic performance ratios increase to a peak, then decrease gradually with increasing Re for different twist ratio. The performance ratio increases with decreasing the twist ratio only in a definite Re range, and the Re range decreases with decreasing twist ratio.     

Modeling of thermal radiation in a participating media with conjugate heat transfer

Modeling a combination of thermal radiation and conjugate heat transfer in a three-dimensional rectangular domain which has a participating media CO₂ flowing through is done numerically in OpenFOAM. The rectangular duct has a vertical step (facing forward to the inlet) which is located at a distance from the inlet (the distance is same as the height of the inlet section). The domain is divided into two regions (namely solid and fluid). Carbon dioxide, a highly absorbing fluid with extinction, is used here as the participating medium. The ability of the code is verified to analyze the thermal radiation in a participating media with conjugate heat transfer. The study was carried out for a constant Reynolds number 250 and a contraction ratio of 0.5. The study focused primarily on the importance of adding thermal radiation on to thermal analysis and the reason behind the Nusselt number variation on different regions of solid–fluid interface. It also discussed the effect of radiative properties, such as optical thickness and linear scattering albedo, on the average convective Nusselt Number.     

Experimental investigation of single phase convective heat transfer coefficient in a corrugated plate heat exchanger for multiple plate configurations

Corrugated plate heat exchangers have larger heat transfer surface area and increased turbulence level due to the corrugations. In this study, experimental heat transfer data are obtained for single phase flow (water-to-water) configurations in a commercial plate heat exchanger for symmetric 30°/30°, 60°/60°, and mixed 30°/60° chevron angle plates. Experiments were carried out for Reynolds number ranging from 500 to 2500 and Prandtl number from 3.5 to 6.5. Experimental results show significant effect of chevron angle and Reynolds number on the heat transfer coefficient. Based on the experimental data, a correlation to estimate Nusselt number as a function of Reynolds number, Prandtl number and chevron angle has been proposed.