Laminar Forced Flow and Heat Transfer in Cross-Corrugated Triangular Ducts

The fluid flow and heat transfer characteristics in a cross-corrugated triangular channel are studied under laminar forced flow and uniform wall temperature conditions. Both the local and the periodic mean values of friction factor and wall Nusselt numbers in the hydro and thermally developing entrance region are investigated. It is found that at higher Reynolds numbers, recirculations in the lower wall valleys are a dominant factor for flow and heat transfer, while at lower Reynolds numbers, parallel flows in the upper wall corrugation are the predominant factor. Compared with a parallel flat plates duct, the Nusselt numbers in a cross-corrugated triangular duct can be enhanced, and can be even higher at higher Reynolds numbers. The growth of steady recirculations and the concomitant periodic disruption and thinning of the boundary layer promote enhanced transport of heat as well as momentum. Effects of heat transfer enhancement are more obvious under higher Reynolds numbers. Two correlations are proposed to predict the periodic mean values of Nusselt numbers and friction factors for Reynolds numbers from 10 to 2000.     

Simulation of free surface MHD‐flow and heat transfer around a cylinder

Magnetohydrodynamic (MHD) free‐surface flow and heat transfer of liquid metal around a cylinder under different Reynolds numbers were simulated numerically. The effects of the application of a magnetic field on wake and vortex shedding were analyzed. The characteristics of flow fields and temperature as well as Lorentz forces under two different Reynolds numbers were presented. The results showed that magnetic field could not only change substantially the flow pattern, but also suppress turbulent viscosity and surface renewal, which degraded heat transfer. Under the same Hartmann numbers, compared with the MHD‐flow and heat transfer of lower Reynolds numbers, the turbulence intensity and interaction between free surface and wake were still stronger for higher Reynolds numbers; consequently, the heat transfer was still high. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 37(1): 11–19, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20189     

Heat Transfer Characteristics and Reynolds Stress Budget of Spatially Advancing Turbulent Flow in a Curved Channel

Direct numerical simulation was performed for the spatially advancing turbulent flow and heat transfer in a two-dimensional curved channel equating the radius ratio to 0.92 or 0.8. The frictional Reynolds number was fixed at 150, whereas the Prandtl number was set at 0.71. According to the numerical result, the remarkable enhancement of heat transfer occurred on the outer wall, suggesting the organized vortex activated the heat transfer. The budgets of Reynolds stresses clarified that the onset and growth of the organized flow was assisted by the direct energy transfer from the mean flow.     

A novel split-dimple interrupted fin configuration for heat transfer augmentation

The use of an interrupted plate fin with surface roughness in the form of split-dimples is investigated. High-fidelity time-dependent calculations are performed for a wide range of Reynolds number ranging from Re_H = 240 to 4000, covering the laminar to fully turbulent flow regimes. The split-dimples provide an additional mechanism for augmenting heat transfer by perturbing continuous boundary layer formation on the fin surface and generating energetic shear layers. High heat transfer regions are observed at the fin and split-dimple leading edges as a result of boundary layer restarts, in regions of flow acceleration between protrusions, and flow impingement on the protrusion surface. While the protruding geometry of the split-dimple also aids in augmenting heat transfer from the fin surface by generating unsteady or turbulent wakes, it also increases pressure losses. The split-dimple fin results in a heat conductance that is 60–175% higher than a plain interrupted plate fin, but at a cost of 4–8 times the frictional losses.     

Laminar forced convection heat transfer to ordered and disordered single rows of cylinders

We study laminar forced convection heat transfer to or from a single row of equidistantly and non-equidistantly spaced parallel cylindrical wires, perpendicular to the flow direction. We report average Nusselt numbers as a function of geometry and flow conditions, for open frontal area fractions between 0.04 and 0.95, Prandtl numbers between 0.7 and 10, and Reynolds numbers (based on the wire diameter and the free stream velocity) between 0.001 and 600. For equidistantly spaced rows of cylindrical wires we propose a general analytical expression for the average Nusselt number as a function of the Reynolds number, Prandtl number and the open frontal area fraction, as well as asymptotic scaling rules for small and large Reynolds. For all studied Prandtl numbers, equidistant rows exhibit decreasing average Nusselt numbers for decreasing open frontal area fractions at low Reynolds numbers. For high Reynolds numbers, the Nusselt number approaches that of a single cylinder in cross-flow, independent of the open frontal area fraction. For equal open frontal area fractions, the Nusselt number in non-equidistant rows is lower than in equidistant rows for intermediate Reynolds numbers. For very low and high Reynolds numbers, non-uniformity does not influence heat transfer.     

波节管纵向逆流换热性能的三维数值模拟研究

本文基于k-ε模型,针对波节管高效换热元件中纵向逆流换热的传热特性和阻力特性进行三维数值模拟研究。传热工质在管程和壳程分别为氦气和氮气,管束采用三角形布置。本文首先分析了不同波距及雷诺数下对换热量影响。为了体现高效换热元件比光管的优越性,随后分析了不同波距及雷诺数对Q/Q0(波节管与光管的换热量比)与Δp/Δp0(波节管与光管的压力降比)。最后得出结论,波距L的增加使高效换热元件的传热性能和阻力性能有所降低,但提高了其综合传热性能。雷诺数的增加会大幅提高换热量,但同时综合传热效率也大幅降低。     

Roughness enhanced mechanism for turbulent convective heat transfer

The mechanism of turbulent convective heat transfer enhancement was experimentally investigated by measuring the heat transfer in two dimensional roughness tubes with different roughness heights at various Reynolds numbers. The results show that there is a maximum Nusselt number ratio (Nu/Nu₀) for a fixed roughness height with increasing Reynolds numbers. For water as working fluid, heat transfer can hardly be increased when the roughness height is lower than the thickness of the viscous sublayer, and both heat transfer and flow friction begin to increase when the roughness height is higher than the viscous sublayer. When the roughness height is more than five times of the viscous sublayer thickness, the flow friction begins to increase sharply but heat transfer is slowly enhanced. So the best heat transfer enhancement for a given pumping power is reached when the roughness height is about three times of the viscous sublayer thickness. The Prandtl number influences to the turbulent heat transfer enhancement by roughness were also analyzed.     

Heat transfer in condensing, pulsating flows

The internal heat transfer coefficient in a pulsating circular pipe flow was determined for both dry and condensing surfaces. The fully-reversing flow was driven by a pulse combustion process at a frequency of 34 Hz. The mean Reynolds numbers ranged from approximately 2600 to 4300, while the instantaneous Reynolds number had a maximum of 18,000. The internal heat transfer is noted to increase by up to a factor of 1.8 due to the pulsating flow prior to the onset of condensation, and by up to 12 times after the onset of condensation. At all Reynolds numbers and flow regimes tested, the flow pulsations were observed to enhance heat transfer when compared to steady flow results.     

Experimental Study on Heat Transfer of Heated Falling Film Under Gas–Liquid Cross-Flow Condition

With the influence of the different gas Reynolds numbers and liquid Reynolds numbers on heated falling film heat transfer, an experiment was performed by noncontact thermal infrared imaging technology under the gas–liquid cross-flow condition. The results indicated that during the increase of liquid Reynolds number the thickness and thermal resistance of liquid film increased in the determined temperature of the heating water, which weakened the heat transfer of the liquid film. However, the increase of liquid Reynolds number strengthened liquid film turbulence and therefore enhanced heat transfer. Under the synergistic effect of these two factors, there should be an optimal liquid Reynolds number that minimizes thermal resistance and maximizes the heat transfer coefficient of the liquid film. Temperature plays an important role in heat transfer of laminar liquid film flow. However, the heat transfer of turbulent liquid film flow is not sensitive to liquid film inlet temperature.     

Transitional flow inside enhanced tubes for fully developed and developing flow with different types of inlet disturbances: Part II–heat transfer

Due to efficiency demands, augmented tubes are often used in heat exchangers with the result that many heat exchangers operate in the transitional region of flow. Due to the paucity of data, however, no data exists for enhanced tubes in this region. This article, being the second of a two-part paper (Part I investigating adiabatic flow), presents experimental heat transfer and diabatic friction factor data for four horizontal enhanced tubes for fully developed and developing flow in the transition region with four different types of inlet geometries. Smooth tube data was used for comparison. It was found that, unlike results obtained for adiabatic flow in Part I, inlet disturbances had no effect on the critical Reynolds numbers, with transition occurring at a Reynolds number of approximately 2000 and ending at 3000. Correlations were developed to predict the heat transfer and friction factors for a wide range of flow regimes, from laminar to turbulent flow. The correlations predicted the heat transfer data on average with a mean absolute error of 9.5%, predicting 85% of the data to within 15%. The friction factor correlations predicted the data with a mean absolute error of 5.5%, predicting 96% of the data to within 20%.     

Thermal and Hydrodynamic Performances of Chaotic Mini-Channel: Application to the Fuel Cell Cooling

Currently, heat exchangers allowing the thermal management of low-temperature fuel cells (PEMFC) are integrated in the bipolar plates and are constituted of a network of straight channels. The flow regime is laminar and thus unfavorable to intense convective heat transfer. In order to increase the power density of the fuel cells, the use of chaotic geometries in the cooling system is envisaged to promote high convective heat transfer. In the present study, several chaotic three-dimensional mini-channels of rectangular cross-section (2 millimeters × 1 millimeter) are evaluated in terms of heat transfer efficiency, mixing properties, and pressure losses. Their performances are compared both to those of the straight channel geometry currently used in the cooling systems of the PEMFC and those of a square-wave mixer. Two Reynolds numbers are considered: 100 and 200. It is shown that a 3-D chaotic channel geometry significantly improves convective heat transfer over that of regular straight or square-wave mixer channels. Of all the geometries studied, one induces higher heat transfer intensification (mean Nusselt number equal to 20) with a strong pressure loss. With an alternative geometry, a better compromise between heat transfer and pressure loss is obtained. However, all of the chaotic geometries present similar mixing rate for the two Reynolds numbers studied.     

垂直环管内旋流换热与流动的数值模拟研究

基于垂直环管内旋流对流动边界层的扰动机理,采用数值模拟的方法研究了叶片角度、雷诺数以及进口水温对管内换热以及流动特性的影响,揭示了重力对环管内旋流流动的内在影响机制。结果表明：与水平环管相比,垂直环管的综合换热性能变化平缓,主要受到重力对压降的影响;与雷诺数相比,叶片角度对流场以及温度场的影响最显著;在雷诺数小于15 000,叶片角度为30°时管内的换热性能最佳。     

Heat Transfer Measurements,Flow Pattern Maps,and Flow Visualization for Non-Boiling Two-Phase Flow in Horizontal and Slightly Inclined Pipe

Local heat transfer coefficients and flow parameters were measured for air-water flow in a pipe in the horizontal and slightly upward inclined (2°, 5°, and 7°) positions. The test section was a 27.9 mm stainless steel pipe with a length to diameter ratio of 100. For this systematic experimental study, a total of 758 data points were taken for horizontal and slightly upward inclined (2°, 5°, and 7°) positions by carefully coordinating the liquid and gas superficial Reynolds number combinations. These superficial Reynolds numbers were duplicated for each inclination angle. The heat transfer data points were collected under a uniform wall heat flux boundary condition ranging from about 1,800–10,900 W/m². The superficial Reynolds numbers ranged from about 740 to 26,000 for water and about 560 to 48,000 for air. A comparison of heat transfer data and flow visualization revealed that the heat transfer results were significantly dependent on the superficial liquid and gas Reynolds numbers, inclination angle, and flow pattern. The experimental data indicated that even in a slightly upward inclined pipe, there is a significant effect on the two-phase heat transfer of air-water flow. Flow pattern maps and flow visualization results for different inclination angles are also presented and discussed.     

HEAT AND FLUID FLOW ACROSS A SQUARE CYLINDER IN THE TWO-DIMENSIONAL LAMINAR FLOW REGIME

The flow structure and heat transfer characteristics of an isolated square cylinder in cross flow are investigated numerically for both steady and unsteady periodic laminar flow in the two-dimensional regime, for Reynolds numbers of 1 to 160 and a Prandtl number of 0.7. The effect of vortex shedding on the isotherm patterns and heat transfer from the cylinder is discussed. Heat transfer correlations between Nusselt number and Reynolds number are presented for uniform heat flux and constant cylinder temperature boundary conditions.     

Comparison between single-phase and two-phases CFD modeling of laminar forced convection flow of nanofluids in a circular tube under constant heat flux

In this article, forced convection heat transfer with laminar and developed flow for water-Al₂O₃ nanofluid inside a circular tube under constant heat flux from the wall was numerically investigated using computational fluid dynamics method. Both single and two-phase models are accomplished for either constant or temperature dependent properties. For this study nanofluids with size particles equal to 100 nm and particle concentrations of 1 and 4 wt% were used. It is observed that the nanoparticles when dispersed in base fluid such as water enhance the convective heat transfer coefficient. The Nusselt number and heat transfer coefficient of nanofluids were obtained for different nanoparticle concentrations and various Reynolds numbers. Heat transfer was enhanced by increasing the concentration of nanoparticles in nanofluid and Reynolds number. Also, a correlation based on the dimensionless numbers was obtained for the prediction the Nusselt number. The modeling results showed that the predicted values were in very good agreement with reference experimental data.     

Heat transfer and flow field in a pipe with sinusoidal wavy surface—I. Numerical investigation

The majority of publications in the field of convective transport enhancement in conduits with wavy walls have provided the distribution of the mean Sherwood or Nusselt number per wavelength. The mechanisms, however, driving the increase in heat and mass transfer have not been clearly understood so far. This paper presents the results of a detailed numerical investigation of local heat and mass transfer enhancement in a pipe with sinusoidally varying diameter, covering a wide range of Reynolds numbers from laminar to turbulent flow. The discussion is focused on the predicted flow field and the turbulence structure, allowing a better understanding of the calculated Sherwood and Nusselt numbers. Part II of this paper deals with the experimental validation of the numerically achieved results.     

Heat Transfer Measurements inside Narrow Channels with Ribs and Trenches

Detailed heat transfer coefficient distributions are obtained for high aspect ratio (width/height = 12.5) duct with rib and trench enhancement features oriented normal to the coolant flow direction. A transient thermochromic liquid crystal technique has been used to experimentally measure heat transfer coefficients from which Nusselt numbers are calculated on the duct surface featuring heat transfer enhancement features. Reynolds number (calculated based on duct hydraulic diameter) ranging from 7100 to 22400 were experimentally investigated. Detailed measurements of heat transfer provided insight into the role of protruding ribs and trenches on the fluid dynamics in the duct. Experimentally obtained Nusselt numbers are normalized by Dittus-Boelter correlation for developed turbulent flow in circular duct. The triangular trenches provide heat transfer enhancement ratios up to 1.9 for low Reynolds numbers. The in-line rib configuration shows similar levels to the trench whereas staggered rib configuration provides heat transfer enhancement ratios up to 2.2 for a low Reynolds number of 7100.     

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.     

Experimental Study of Forced Convection Heat Transfer of Water in a Short Microduct at a Turbulent Reynolds Number

The present study deals with experimental investigation of cooling of machining tools, by water flowing through a microduct at the tip of the tool. The average diameter of the microduct is 200 μm and the flow takes place at a turbulent Reynolds number. The outer wall temperature of the tool and the temperature of water at inlet and exit have been measured. The convective heat transfer coefficient is calculated at different wall temperatures and mass flux. The experimental results show that the average Nusselt numbers in the short microduct are higher than those predicted by conventional correlations for large-diameter ducts. This enhancement may be attributed to the micro size of the duct, entry effects, transition from laminar to turbulent flow at the microduct entrance, suspended microscopic particles in coolant water, and Prandtl number estimation based on the mean fluid temperature. A correlation has been proposed to compute convective heat transfer during turbulent flow through a short microduct of a particular geometry for a range of Reynolds and Prandtl numbers.