Numerical Predictions of Laminar Forced Convection Heat Transfer with and without Buoyancy Effects from an Isothermal Horizontal Flat Plate to Supercritical Nitrogen

Numerical predictions are made for Laminar Forced convection heat transfer with and without buoyancy effects for Supercritical Nitrogen flowing over an isothermal horizontal flat plate with a heated surface facing downwards.Computations are performed by varying the value ofΔT from5 to 30 K and P_(∞)/P_(cr)ratio from1.1 to 1.5.Variation of all the thermophysical properties of supercritical Nitrogen is considered.The wall temperatures are chosen in such a way that two values of Tw are less than T∗(T*is the temperature at which the fluid has a maximum value of Cp for the given pressure),one value equal to T∗and two values greater than T∗.Three different values of U∞are used to obtain Re∞range of 3.6×10_(4)to 4.74×10^(5)for forced convection without buoyancy effects and Gr_(∞)/Re^(2)_(∞)range of 0.011 to 3.107 for the case where buoyancy effects are predominant.Six different forms of correlations are proposed based on numerical predictions and are compared with actual numerical predictions.It has been found that in all six forms of correlations,the maximum deviations are found to occur in those cases where the pseudocritical temperature TT∗lies between the wall temperature and bulk fluid temperature.

Optimal selection of annulus radius ratio to enhance heat transfer with minimum entropy generation in developing laminar forced convection of water-Al2O3 nanofluid flow

Heat transfer and entropy generation of developing laminar forced convection flow of water-Al_2O_3 nanofluid in a concentric annulus with constant heat flux on the walls is investigated numerically. In order to determine entropy generation of fully developed flow, two approaches are employed and it is shown that only one of these methods can provide appropriate results for flow inside annuli. The effects of concentration of nanoparticles, Reynolds number and thermal boundaries on heat transfer enhancement and entropy generation of developing laminar flow inside annuli with different radius ratios and same cross sectional areas are studied. The results show that radius ratio is a very important decision parameter of an annular heat exchanger such that in each Re, there is an optimum radius ratio to maximize Nu and minimize entropy generation. Moreover, the effect of nanoparticles concentration on heat transfer enhancement and minimizing entropy generation is stronger at higher Reynolds.

Forced Convective Heat Transfer Experiment of Supercritical Water in Different Diameter of Tubes

Forced convective experiment of supercritical water was performed in Inconel-625 tubes of 4.62 mm, 7.98 mm and 10.89 mm in diameter. The water flowed upward, covering the ranges of pressure of 23.4 MPa to 25.8 MPa, mass flux of 90 kg/m^2s to 3,281 kg/m^2s, local bulk temperature of 102-384 ℃, inner wall temperature of 167-669℃ and heat flux of up to 2.41 MW/m^2. The results exhibited severe deteriorated and enhancement heat transfer. The experimental results can be calculated by the Jackson＇s correlation and the Bishop＇s correlation mostly. But some data with strong effects of the buoyancy force and the variations of flow regimes can not be predicted properly.

Turbulent forced convection in a heat exchanger square channel with wavy-ribs vortex generator

Turbulent forced convective heat transfer and flow con figurations in a square channel with wavy-ribs inserted diagonally are examined numerically. The in fluences of the 30° and 45° flow attack angles for wavy-ribs, blockage ratio, R B= b/H = 0.05–0.25 with single pitch ratio, R P= P/H = 1 are investigated for the Reynolds number based on the hydraulic diameter of the square channel, Re = 3000–20000. The use of the wavy-ribs, which inserted diagonal in the square channel, is aimed to help to improve the thermal performance in heat exchange systems.The finite volume method and SIMPLE algorithm are applied to the present numerical simulation. The results are presented on the periodic flow and heat transfer pro files, flow con figurations, heat transfer characteristics and the performance evaluations. The mathematical results reveal that the use of wavy-ribs leads to a higher heat transfer rate and friction loss over the smooth channel. The heat transfer enhancements are around 1.97–5.14 and 2.04–5.27 times over the smooth channel for 30° and 45° attack angles, respectively. However, the corresponding friction loss values for 30° and 45° are around 4.26–86.55 and 5.03–97.98 times higher than the smooth square channel, respectively. The optimum thermal enhancement factor on both cases is found at R B= 0.10 and the lowest Reynolds number, Re = 3000, to be about 1.47 and 1.52, respectively, for 30° and 45° wavy-ribs.

Thermal performance and entropy generation for nanofluid jet injection on a ribbed microchannel with oscillating heat flux: Investigation of the first and second laws of thermodynamics

In current numerical study,forced flow and heat transfer of water/NDG(Nitrogen-doped graphene)nanofluid in nanoparticles mass fractions(φ)of 0,2%and 4%at Reynolds numbers(Re)of 10,50,100 and 150 are simulated in steady states.Studied geometry is a two-dimensional microchannel under the influence of nanofluid jet injection.Temperature of inlet fluid equals with Tc=293 K and hot source of microchannel is under the influence of oscillating heat flux.Also,in this research,the effect of the variations of attack angle of triangular rib(15°,30°,45°and 60°)on laminar nanofluid flow behavior inside the studied rectangular geometry with the ratio of L/H=28 and nanofluid jet injection is investigated.Obtained results indicate that the increase of Reynolds number,nanoparticles mass fraction and attack angle of rib leads to the increase of pressure drop.By increasing fluid viscosity,momentum depreciation of fluid in collusion with microchannel surfaces enhances.Also,the increase of attack angle of rib at higher Reynolds numbers has a great effect on this coefficient.At low Reynolds numbers,due to slow motion of fluid,variations of attack angle of rib,especially in angles of 30°,45°and 60°are almost similar.By increasing fluid velocity,the effect of the variations of attack angle on pressure drop becomes significant and pressure drop figures act differently.In general,by using heat transfer enhancement methods in studied geometry,heat transfer increases almost 25%.

Temperature rise of He Ⅱ forced flow and its negative Joule-Thomson effect

The temperature rise of He Ⅱ transfer system due to the negative Joule-Thomson(JT)effect is one of the major problems in the He Ⅱ forced flow system design.Negative Joule-Thomson effect of the He Ⅱ forced flow was analyzed and calculated in this paper.The temperature rise due to the heat leak along the transfer pipeline was calculated by the simplified equation and was modified by considering the negative Joule-Thomson effect.The modified results were compared with the temperature rise obtained by non-linear differential equations with consideration of the pressure gradient.The results show that the pressure gradient has strong effect on the temperature distribution.The modified results are in good agreement with the values calculated by the complicated equation,which verifies the effectiveness of the simplified equation in calculating the temperature rise when the negative JT effect of He Ⅱ is known.

Study on Forced Convective Heat Transfer of Non-Newtonian Nanofluids

This paper is concerned with the forced convective heat transfer of dilute liquid suspensions of nanoparticles （nanofluids） flowing through a straight pipe under laminar conditions. Stable nanofluids are formulated by using the high shear mixing and ultrasonication methods. They are then characterised for their size, surface charge,thermal and rheological properties and tested for their convective heat transfer behaviour. Mathematical modelling is performed to simulate the convective heat transfer of nanofluids using a single phase flow model and considering nanofluids as both Newtonian and non-Newtonian fluid. Both experiments and mathematical modelling show that nanofluids can substantially enhance the convective heat transfer. Analyses of the results suggest that the non-Newtonian character of nanofluids influences the overall enhancement, especially for nanofluids with an obvious non-Newtonian character.

Second Law Analysis of the Optimal Fin by Minimum Entropy Generation

Based on the entropy generation concept of thermodynamics, this paper estabfished a general theoretical model for the analysis of entropy generation to optimize fins, in which the minimum entropy generation was selected as the object to be studied. The irreversibility due to heat transfer and friction was taken into account so that the minimum entropy generation number has been analyzed with respect to second law of thermodynamics in the forced cross-flow. The optimum dimensions of cylinder pins were discussed. It＇s found that the minimum entropy generation number depends on parameters related to the fluid and fin physical parameters. Varlatioms of the minimum entropy generation number with different parameters were analyzed.

Performance index improvement of a double-pipe cooler with MgO/water-ethylene glycol (50:50) nano-suspension

A series of tests was conducted to unlock the potential application of MgO/water-EG(ethylene glycol)nanofluids(NF)in a double-pipe heat exchanger(HEX).The overall heat transfer coefficient(HTC),the inlet temperature of the working fluid,the fluid pressure drop(FPD),friction factor(FF)and the hydraulic performance index of the NF within the HEX were experimentally measured.Fouling of nanoparticles(NPs)within the Hex was also studied and modelled using asymptotic particulate fouling model.Results showed that MgO NPs can enhance the HTC by 39%at Re=10500 and wt.%=0.3 in the turbulent regime.Also,the presence of MgO NPs augmented the FF and the FPD values.The former was enhanced 33.8%,while the latter was augmented by 37%both at wt.%=0.3 and at Reynolds number=10500.Results also revealed that the formation of porous particulate fouling layer on the internal wall of the inner tube creates a fouling thermal resistance which changes asymptotically with time.Overall,MgO/water-ethylene glycol shows a great potential to be used as a coolant within a HEX.