The dispersion of nanoparticles in conventional heat transfer fluids has been proven to improve the performance of the fluids. However, the study on the heat transfer performance of hybrid nanofluids in the mixture of water and green bioglycol (BG) is limited in the literature. This paper presents the heat transfer performance and friction factor of green BG-based TiO 2 –SiO 2 nanofluids. The TiO 2 and SiO 2 nanoparticles were dispersed in the mixture of 60:40 water: bioglycol (W/BG) and prepared at various concentrations up to 2.5% and composition ratios of 20:80. The experimental study on forced convection heat transfer was done under turbulent flow at constant heat flux for operating temperature of 70 °C. The heat transfer enhancement increased significantly with volume concentrations. The maximum heat transfer enhancements of the TiO 2 –SiO 2 nanofluids at bulk temperature of 70 °C were observed to be up to 67.81% for 2.5% volume concentration. A slight friction factor escalation of the nanofluids was observed with 12% maximum increment. New correlations were developed to estimate the Nusselt number, and friction factor with average deviations of less than 4.3%. As a conclusion, the employment of the ecofriendly coolant nanofluids in improving thermal performance is proven and applicable for turbulent forced convection heat transfer applications.

The impact of measurement uncertainty on Nusselt number correlations for supercritical CO 2 is investigated. Selection of appropriate reference quantities for these correlations, such as thermal conductivity, appears to have a large effect on accurately predicting heat transfer rates. Supercritical CO 2 work heavily depends on real fluid property data, which show that fluid properties have very large gradients with respect to temperature and pressure near the critical point. The sharp gradients imply heat transfer predictions are highly sensitive to the accuracy of temperature and pressure experimental measurements in this region. Root sum of squares uncertainties at various property values indicate predictably large (on the order of 1000%) uncertainties in calculated Reynolds, Prandtl, and Nusselt numbers near the critical point. Interestingly, uncertainties remain several times the calculated value for operating pressures between 7.5 and 8.5 MPa, which are common in the experimental literature—highlighting a need for careful application of correlations near the pseudocritical line and the benefits of presenting dimensional data in the literature.

Cattaneo–Christov heat flux model was proposed to remedy the weakness of the traditional Fourier heat flux model to maintain the finite travel time of heat. The literature is replete with numerical studies to understand the heat transfer enhancement property. The present effort is to provide a mathematical rigor and to analytically demonstrate why the new model should act toward cooling and thus, in the way of enhancing the heat transfer rate from the surfaces. The derived and presented formulae here prove this assertion through comparison with a few selected examples from the open literature.

Wall radiative heat transfer in inner straight fin tubes is very complex considering the coupling of heat conduction in fins and radiative heat transfer of medium with solid surfaces, influenced by a number of factors such as fin parameters, radiative properties and run conditions. The method of simulation is very inconvenient to design the heat exchanger involving the enhancement of inner straight fins on wall radiative heat transfer in tube, such as ascension pipe heat exchanger and radiant syngas cooler. In this study, a simplified method is used. The average radiative heat transfer between radiative medium and solid surfaces is first studied by simulation with fins supposed have a constant temperature. Then an approximate correlation of this radiative heat transfer coefficient is proposed using the traditional radiative heat transfer calculation method with a view coefficient, which has a little error within 15%. A calculation method of average wall radiative heat transfer coefficient is further derived based on fin theory with an average temperature of fin surface used to consider the varying of the temperature along the fin when the conductivity of fins is finite. Using the predicting method proposed, a method for design calculation of fins in tubes to optimize wall radiative heat transfer is also given with three dimensionless numbers of π/n, 2H/D, and nt/πD representing the structural properties of fins defined. Three cases of (k s = 43 W/(m•K), ɛ w = 0.7, T g = 1073.15 K, T w = 573.15 K, D = 0.66 m, κD = 1.32), (k s = 162 W/(m•K), ɛ w = 0.7, T g = 1073.15 K, T w = 573.15 K, D = 0.66 m, κD = 1.32) and (k s = 43 W/(m•K), ɛ w = 0.3, T g = 973.15 K, T w = 673.15 K, D = 0.66 m, κD = 0.66) are analyzed in detail based on the design calculation method. It is verified that the radiative heat transfer could be enhanced twice by introducing fins. Under the same h 0 , conductivity and emissivity are two important factors that should be considered to choose the material for fins. The microfins or the special treatments on the tube wall are a best choice for the fin material having a relatively small conductivity.

Heat transfer analysis of liquid–liquid Taylor flow in previous studies almost never shows the effect of slug length on heat transfer. The homogenous or single-phase analysis is often the only method available to deal with flow of multicomponents. In this study, a new approach is developed to model the segmented liquid–liquid Taylor flow as two separated systems to present heat transfer enhancement for each component. The effect of internal circulation and boundary layer renewal within the two fluid components is clearly observed. An experimental setup was assembled using an open loop system at miniscale size in which liquid–liquid Taylor flow is heated under a constant wall temperature. Three silicone oils of 1, 3, and 5 cSt were segmented using distilled water at three volume fractions 0.25, 0.5, and 0.75. Finally, heat transfer data of the dimensionless mean wall heat flux shows good agreement with a predictive model proposed in an earlier work by the second author. The results show an impact of the fluid cell dimensions on the rate of heat transfer.

Dimple on the surface is widely used in electronic cooling equipment, turbine blades, and combustion chamber gaskets and so on, which is a good structure for heat transfer enhancement. In this paper, taking comprehensive performance parameters of flow and heat transfer PEC as an evaluation parameter, numerical simulation, and multi-island genetic algorithm are combined to optimize the shape of the dimple in microchannel under fully developed laminar condition. The results show that the optimal dimple is asymmetric along the flow direction, and the deepest position of which shifts downstream, which is dependent on the Reynolds number, the dimple diameter, and the periodic length. With the increase of the Reynolds number and the dimple diameter, the Nusselt number ratio, the Fanning fraction factor ratio, and the comprehensive performance parameter PEC increase for the optimal dimple. The separation of the fluid in the front edge of dimple is not conducive to heat transfer. The number and size of the vortex, the impact, and the reattachment are found to be the key factors affecting the heat transfer in the dimple. As the periodic length L of the heat transfer unit decreases, the heat transfer is enhanced and the flow resistance increases, and the comprehensive performance of the microchannel becomes better.

The slit pillar allows a small fraction of the mainstream flow through pillar to disturb the pillar wake zone fluid and eventually enhance the local and global heat transfer performances in microchannels. In this study, three-dimensional full-domain numerical simulations on the hydrodynamic and thermal characteristics of slit pillar array in microchannels are performed. Effects of slit angle and height over diameter (H/D) ratio on the fluid flow and heat transfer are studied. Comparisons with the nonslit pillar array are conducted on pressure drop, surface temperature, Nusselt number, and thermal performance index (TPI). Furthermore, the results are analyzed by using the entropy generation. As a result of secondary flows and enhanced convective heat transfer area, all cases at H/D ratio of 0.3 demonstrate enhanced heat transfer performance at an increase of 18.0–34.7% on Nusselt number, while a reduction of 3.4–12.9% on pressure drop in comparison to the criterion case at the same conditions. Among them, slit 15–15 deg shows the best comprehensive heat transfer performance. Due to the improved uniformities of velocity and temperature distributions, all slit pillar array microchannels show decreased entropy generation. The maximum entropy generation reduction can reach up to 15.8%, as compared with the criterion case at the same conditions. The above results fully demonstrate that the novel slit pillar array microchannel heat sink can be used as an effective approach for heat transfer enhancement.

Tube receiver with central corrugated insert was introduced as the absorber tube of parabolic trough receiver to increase the overall heat transfer performance of the tube receiver for parabolic trough solar collector (PTC) system. The Monte Carlo ray tracing method (MCRT) coupled with finite volume method (FVM) was adopted to investigate flow characteristics and the heat transfer performance of tube receiver for parabolic trough solar collector system. The numerical results were successfully validated with the empirical correlations existing in the literature. The numerical results indicated that the introduction of the corrugated insert inside the absorber tube of the parabolic trough receiver can effectively enhance the heat transfer performance, where the average Nusselt number can be increased up to 3.7 times compared to the smooth absorber. While the overall heat transfer performance factor can be found to be in the range of 1.3–2.6. The results indicate that the heat transfer increases with increasing corrugated insert twist ratio and increases also with decreasing pitch between two corrugations.

This research effort is related to the detailed analysis of the temporal evolution of thermal boundary layer(s) under periodic excitations. In the presence of oscillations, the nonlinear interaction leads to the formation of secondary flows, commonly known as acoustic streaming. However, the small spatial scales and the inherent unsteady nature of streaming have presented challenges for prior numerical investigations. In order to address this void in numerical framework, the development of a three-tier numerical approach is presented. As a first layer of fidelity, a laminar model is developed for fluctuations and streaming flow calculations in laminar flows subjected to traveling wave disturbances. At the next level of fidelity, two-dimensional (2D) U-RANS simulations are conducted across both laminar and turbulent flow regimes. This is geared toward extending the parameter space obtained from laminar model to turbulent flow conditions. As the third level of fidelity, temporally and spatially resolved direct numerical simulation (DNS) simulations are conducted to simulate the application relevant compressible flow environment. The exemplary findings indicate that in certain parameter space, both enhancement and reduction in heat transfer can be obtained through acoustic streaming. Moreover, the extent of heat transfer modulations is greater than alterations in wall shear, thereby surpassing Reynolds analogy.

This study presents the results of the experimental study on hydrodynamics and heat transfer in separation zone in front and behind a single rectangular perforated rib mounted on a flat plate. The effects of perforation open-area ratio ( β = 12%, 23.5%, and 44%) and the location of the hole on the rib (at the bottom, at the top, and in a staggered arrangement) on the mean and fluctuating characteristics of the turbulent dynamic and thermal boundary layers in the median section of the plate were examined. It was established that the stagnant and recirculation zones in the front and behind the perforated rib were shifted and became smaller or disappeared.

The effects of nanoferrofluids on the overall performance of curved tubes (with various radii of curvature) are experimentally investigated under the influence of constant and alternating magnetic fields. The working fluids are distilled water and a ferrofluid (Fe 3 O 4 /water) with 0.2% and 0.4% volume concentrations. The experiments are performed under a constant wall heat flux (≈12,700 W/m 2 ) using a chrome–nickel electric heater element insulated by refractory fabrics. The mass flowrate is varied from 0.2 to 0.7 kg/min. There are three key parameters, namely, type of the magnetic field, volume of concentration of nanoparticles, and radius of curvature of the pipes that affect the hydrodynamic and thermal characteristics of the system, but the latter is comparatively the dominant factor. If the Reynolds number is 930 in the pipe of 0.2-m diameter of curvature, and also a 50-Hz alternating magnetic field is applied to the curved pipe, the results reveal that using a 0.4% ferrofluid, Nusselt number is improved by 32% compared to that of the distilled water. Nevertheless, due to the undesirable influence of pressure gradient, the best overall effectiveness of 1.12 is attained in the circumstances but in the pipe of 0.4-m diameter of curvature.

In this study, a three-dimensional numerical investigation on the thermohydrodynamic performance of a recently proposed recharging microchannel (RMC) is carried out. In this design, a straight microchannel is split into more than one smaller length channels (having individual inlet and outlet) placed end to end. This design enhances overall heat transfer and maintains temperature uniformity across the substrate length. The comparison of fluid flow and heat transfer performance of RMC, interrupted microchannel (IMC) and straight microchannel (SMC) with the same hydraulic diameter and substrate length are presented to explore the effect of geometrical configuration on heat transfer enhancement. The parametric variations include the number of channels (n), transverse wall length (L tw ), channel aspect ratio ( α ), and flow Reynolds number. The results reveal that recharging microchannel shows better thermal performance compared to simple and interrupted microchannel with a maximum performance factor of 1.80. The results also indicate that the performance factor of RMC increases with an increase in the number of small channels, transverse wall length, and channel aspect ratio. The outcome of this study indicates the possible use of recharging microchannel heat sinks for high heat flux removal applications such as electronic cooling.

Pillar microchannel heat sinks have been widely used for chip cooling, while their overall heat transfer performance is restricted by the stagnation flow in pillar wake zone. In this work, a simple but effective method using slit microstructure modified on pillar was proposed to enhance wake zone heat transfer. It enables a special flow path for the incoming fluid that intensively disturbs the wake fluid. To validate the proposed method, a three-dimensional simulation was employed to study the laminar flow and heat transfer characteristics in the slit pillar microchannel. The pillar without slit design was also investigated for comparative analysis. Effects of slit angle ( θ ), height over diameter ratio ( H/D ), and blocking ratio ( D/W ) of a single pillar were systematically studied at the Reynolds numbers of 26–260. Results showed the case with θ = 0 deg always demonstrated lower surface temperature, higher Nusselt number and higher thermal performance index (TPI) compared to other cases with different slit angles at the same conditions. Furthermore, it was interesting to find that the slit configuration was not suitable for long pillar microchannel, but preferred for high blocking ratio pillar microchannel at present ranges ( H/D ≤ 1, D/W ≤ 0.5). The slit pillar array microchannel was also explored and observed with improved overall heat transfer performance. The proposed slit microstructure well prevents the heat transfer deterioration in pillar wake zone, which is promisingly to be used for cooling performance improvement of electronic device.

The design of Ranque–Hilsch vortex tube (RHVT) seems to be interesting for refrigeration and air conditioning purposes in industry. Improving thermal efficiency of the vortex tubes could increase the operability of these innovative facilities for a wider heat and cooling demand to this end; it is of an interest to understand the physical phenomena of thermal and flow patterns inside a vortex tube. In this work, the flow phenomena and the thermal energy transfer in RHVT are studied for three RHVT: straight, divergent, and convergent vortex tubes. A three-dimensional numerical analysis of swirling or vortex flow is performed, verified, and validated against previous experimental and numerical data reported in literature. The flow field and the temperature separation inside an RHVT for different configuration of straight, five angles of divergent hot tube (1 deg, 2 deg, 3 deg, 4 deg, and 6 deg) and five angle of convergent hot tube (0.5 deg, 0.8 deg, 1 deg, 1.5 deg, and 2 deg) are investigated. The thermal performance for all investigated RHVTs configuration is determined and quantitatively assessed via visualizing the stream lines for all three scenarios.

In order to ensure flight safety in cold winter, aircraft ground deicing is crucial and necessary. In Chinese deicing fluid heating system, the helically coiled tube is paramount exchanger to heat deicing fluid. The deicing fluid is ethylene-glycol-based mixture with high viscosity. Aiming at heat transfer enhancement of deicing fluid, ring rib is formed by an embossed tube wall toward the internal of the tube; thus, transversely corrugated helically coiled tube (TCHC) is achieved. Depth and width are two key geometrical parameters of ring rib. Based on field synergy principle, the influence of depth–diameter ratio (H/D) and width-diameter ratio (w/D) is investigated through numerical simulation. The results show that outlet temperature, mean convection heat transfer coefficient, and Nusselt number have similar trends, which first increase and then decrease nonlinearly. The variation of flow resistance coefficient is inversely proportional to Reynolds number. Especially, the effect of H/D is more significant than that of w/D. Field synergy angle and velocity field are also analyzed to reveal the mechanism of heat transfer. TCHC performs better than the original tube. Orthogonal experiment calculates the outlet temperature of TCHC when H/D and w/D change. The combination of H / D = 0.075 and w / D = 0.5 is best solution. TCHC effectively enhances heat transfer of deicing fluid. Therefore, TCHC is beneficial to improve the deicing efficiency and ensure the flight punctuality.

Recently, energy saving problem attracts increasing attention from researchers. This study aims to determine the optimal arrangement of a tube bundle to achieve the best overall performance. The multi-objective genetic algorithm (MOGA) is employed to determine the best configuration, where two objective functions, the average heat flux q and the pressure drop Δp, are selected to evaluate the performance and the consumption, respectively. Subsequently, a decision maker method, technique for order preference by similarity to an ideal solution (TOPSIS), is applied to determine the best compromise solution from noninferior solutions (Pareto solutions). In the optimization procedure, all the two-dimensional (2D) symmetric models are solved by the computational fluid dynamics (CFD) method. Results show that performances alter significantly as geometries of the tube bundle changes along the Pareto front. For the case 1 (using staggered arrangement as initial), the optimal q varies from 2708.27 W/m 2 to 3641.25 W/m 2 and the optimal Δp varies from 380.32 Pa to 1117.74 Pa, respectively. For the case 2 (using in-line arrangement as initial), the optimal q varies from 2047.56 W/m 2 to 3217.22 W/m 2 and the optimal Δp varies from 181.13 Pa to 674.21 Pa, respectively. Meanwhile, the comparison between the optimal solution with maximum q and the one selected by TOPSIS indicates that TOPSIS could reduce the pressure drop of the tube bundle without sacrificing too much heat transfer performance.

Heat transfer from a cylinder of square cross section (either dissipating constant heat flux (q W ) or maintaining at a constant temperature (T W )) placed near a plane wall under the incidence of nonuniform linear/nonlinear velocity profile is studied numerically (finite volume method (FVM), quadratic upstream interpolation for convective kinematics (QUICK), and SIMPLE). The conventional fluids are chosen as water, and ethylene glycol–water mixture. The nanoparticles are selected as Al 2 O 3 and CuO. Roles of pressure gradient P (at the inlet), temperature of base fluids, thermal conditions (T W or q W ), and nanofluids' parameters (nanoparticle concentrations ( ϕ ), diameter, materials, and base fluids) on the heat transfer (Nusselt number ( Nu ¯ M )) of the cylinder are investigated here. Nu ¯ M enhancement from the cylinder together with its drag coefficient reduction/increment due to addition of nanomaterials in both fluids at two different temperatures is assessed under the Couette flow. Classical fluid dynamics relationship among Nu ¯ M , Reynolds number (Re), and Prandtl number is discussed through Colburn j–factor, and hence the utility of proposed correlation between j–factor and Re toward engineering problems is also explored. The graphical observations of dependency of Nu ¯ M on the aforesaid parameters are reconfirmed by proposed functional forms of Nu ¯ M = Nu ¯ M ( P ) , Nu ¯ M = Nu ¯ M ( ϕ ) and hence Nu ¯ M = Nu ¯ M ( P , ϕ ) . An effort is made to examine the effectiveness of the aforementioned parameters on the heat transfer enhancement rate.

Numerical simulations are used to analyze the thermal performance of turbulent flow inside heat exchanger tube fitted with cross-cut twisted tape with alternate axis (CCTA). The design parameters include the Reynolds number ( 5000 < Re < 15 , 000 ) , cross-cut width ratio ( 0.7 < b / D < 0.9 ) , cross-cut length ratio ( 2 < s / D < 2.5 ) , and twist ratio ( 2 < y / D < 4 ) . The objective functions are the Nusselt number ratio ( Nu / Nu s ) , the friction factor ratio ( f / f s ) , and the thermal performance ( η ) . Response surface method (RSM) is used to construct second-order polynomial correlations as functions of design parameters. The regression analysis shows that heat transfer ratio decreased with increasing both the Reynolds number and the width to diameter ratio of the twisted tape. This means that the twisted tape has more influence on heat transfer at smaller inlet fluid velocities. Sensitivity analysis reveals that among the effective input parameters, the sensitivity of Nu / Nu s to the Reynolds number is the highest. The results reveal that thermal performance enhances with increasing the width to diameter ratio of the twisted tape ( b / D ) . The maximum thermal performance factor of 1.531 is obtained for the case of Re = 5000 , b / D = 0.9 , s / D = 2.5 , and y / D = 4 .

Vortex generator as secondary flow enhancement technique has captured the attention of many researchers recently to augment the performance of the fin-and-tube heat exchanger (FTHE). There are various vortex generator parameters that influence the thermal and hydraulic performance in the FTHE such as the geometry and arrangement. In this study, the effect of different vortex generator geometries and arrangements was investigated using numerical simulation method. There are three vortex generator geometries studied including rectangular winglet (RWVG), delta winglet (DWVG), and trapezoidal winglet (TWVG). The vortex generators were placed behind tubes either in common flow down (CFD) or common flow up (CFU) arrangement. The introduction of vortex generators behind tubes resulted in heat transfer augmentation but comes together with higher pressure drop penalty. Further analysis on the thermal performance has found that TWVG in CFU arrangement almost obtained similar thermal performance factor with respect to the baseline case at Reynolds number 500 and 600. However, the thermal performance factor for TWVG in CFU arrangement decreases as the Reynolds number further increased. For other vortex generator cases, lesser thermal performance factor was found as compared to the baseline case.

Laminar forced convection heat transfer from a constant temperature tube wrapped fully or partially by a metal porous layer and subjected to a uniform air cross-flow is studied numerically. The main aim of this study is to consider the thermal performance of some innovative arrangements in which only certain parts of the tube are covered by metal foam. The combination of Navier–Stokes and Darcy–Brinkman–Forchheimer equations is applied to evaluate the flow field. Governing equations are solved using the finite volume SIMPLEC algorithm and the effects of key parameters such as Reynolds number, metal foam thermophysical properties, and porous layer thickness on the Nusselt number are investigated. The results show that using a tube which is fully wrapped by an external porous layer with high thermal conductivity, high Darcy number, and low drag coefficient, can provide a high heat transfer rate in the high Reynolds number laminar flow, increasing the Nusselt number almost as high as 16 times compared to a bare tube. The most important result of thisstudy is that by using some novel arrangements in which the tube is partially covered by the foam layer, the heat transfer rate can be increased at least 20% in comparison to the fully wrapped tube, while the weight and material usage can be considerably reduced.