ABSTRACTDirect absorption solar collectors (DASCs) typically achieve high efficiency due to the volumetric heat absorption process facilitated by the working fluids. In this study, carbon black (CB) nanofluids were utilized as the working fluid to experimentally and numerically investigate the thermal performance of a rectangular DASC. The findings suggest that the nanoparticles have the potential to enhance the efficiency of the DASC.Direct absorption solar collectors (DASCs) are known for their high efficiency, which is achieved through the volumetric heat absorption process provided by the working fluids In this study, carbon black (CB) nanofluids were used as these working fluids to study the thermal performance of a rectangular DASC. The experiments were conducted using water and nanofluids with 0.05 wt.% nanoparticle concentration, at different flow rates and tilt angles, under a concentrated simulated solar power source. Our results show that the efficiency of the DASC increased as the flow rate increased. The DASC was more efficient when the receiving surface was facing downwards (tilt angle of 0°), and the efficiency was 35% higher than when the receiving surface was facing upwards (tilt angle of 180°). A computational fluid dynamics (CFD) model, which was validated against our experimental results, analyzed the DASC performance under different CB concentrations. According to the simulations, the highest efficiency occurred at a concentration of 0.05 wt.%. The study also highlighted the distribution of temperature and velocity of the nanofluids, as well as the volume fraction of carbon black during the flow process.

ABSTRACTThis paper presents an experimental inverse approach to simultaneously estimate the temperature-dependent thermal properties of AISI 316 stainless steel. The heat flux conducted to the sample was corrected by using a transducer, evaluating contact resistance at microscopic level, and performing experiments under vacuum. Estimates were severely biased when heat flux correction was not applied. Metaheuristics were used to enlarge the search space, which can cover the thermal properties of all known metallic materials, thereby taking advantage of better thermal modeling adequacy. The estimation reliability was shown by comparison with literature data, statistical significance, uncertainty analysis, and inverse heat flux retrieval.

ABSTRACTThe solar air heater (SAH), which utilizes solar energy to satisfy the building’s heating demands, is an important integrated component of the building. However, SAH’s performance isn’t good enough for it to be used in building. Adding roughness component to the effective zone of heat transfer underneath the absorber plate is the most successful and efficient technique to augment the thermal performance of solar air heater (SAH). In this regards, artificial roughness consisting of several Multiple Open Trapezoidal Rib (MOTR) has been devised with intention of capitalizing on the benefits of secondary flows with regard to the improvement of heat transmission. The geometrical parameters of MOTRs such as relative limb length (L/w = 0.2–1.0), Reynold number (Re = 2000–16000), relative rib height (e/Dh = 0.023–0.054), and relative roughness pitch (p/e = 6–12) have been exploited, while relative width ratio (W/w = 6) and angle of attack (α = 60°) are maintained constant in all the set of experiments. The highest augmentation in Nusselt number (Nu) is observed as 6.07 times as that of smooth absorber plate. In addition, following empirical correlations have been developed for Nu & f with accuracy of average absolute deviation of 5.47% and 5.02%, respectively.

ABSTRACTIncreasing processing capacity without modifying the size of electronic devices has made thermal management important in the electronic industry. Commercialized thermal management, such as in a conventional air cooling system, is insufficient for electronic devices with high heat flux dissipation. Pool boiling is a better method for heat transfer because it can dissipate a substantial amount of heat at low wall superheats. This study focused on heat transfer enhancement using passive approaches, including nanostructures and microporous sintered surfaces over open microchannel surfaces and microchannels with pin-fins. In the present work, seven structures were studied in pool boiling, wherein experiments elucidated the effects of microchannels, sintered, and pin fins (micropillar) on boiling heat transfer from a copper chip in a pool of degassed water. Boiling performance is ascertained via critical heat flux (CHF) and heat transfer coefficient (HTC). The best heat transfer performance showed a heat flux of 243.75W/cm2 at 15.46°C on the pin-fins chip, which was 1.9 times the heat flux of the plain chip. The highest HTC was 181.03 kW/(m2 oC) at a heat flux of 172.61 W/cm2 for the microchannel with single pin-fins. The HTC enhancement was 2.8 times greater than the plain surface. It was found experimentally that HTC and CHF improved on all modified surfaces compared to the plain copper chip baseline.

ABSTRACTIn order to have a heat transfer per volume, heat exchangers have to have high overall heat coefficient. The effects of a transverse wedge on the heat transfer, the pressure drop, and the thermal efficiency factor (TEF) of air flow through a tube bundle heat exchanger was investigated. TEF increases continuously with the wedge aspect ratio (α). Form α = 0.275, 0.550, 0.758 to 1, TEF increases continuously from 1.157 to 1.84, 1.11 to 1.72, 1.05 to 1.42, and 1.02 to 1.39 for x/L = 1/3, 2/3, 1, and 0, respectively.

ABSTRACTThe work focuses on developing a detailed understanding of the effects of initial composition (Cin) and bottom cooling temperatures (TB) on different characteristics of double-diffusive convection during unidirectional solidification of water-NH4Cl solution in a complete non-intrusive manner. The qualitative investigation, simultaneous quantification of transported parameters (composition and temperature) and fluid velocities associated with flow patterns are carried out using rainbow schlieren deflectometry, dual-wavelength interferometry and PIV (particle image velocimetry) technique respectively. The dependence of the characteristics of double-diffusive convection for different Cin and TB is explained on the basis of real-time whole-field investigation using a combination of aforementioned imaging techniques.

ABSTRACTThe novelty of microencapsulated phase change material slurry (MPCMS) in the microchannel is a proven technique for enhancing the heat transfer due to its combination of high latent heat transfer and surface area. In this experimental investigation, the paraffin is encapsulated with silica and titanium shells, and its thermal performance is compared with three different configurations like straight, dimple-cavity, and rib-groove microchannel heat sinks. The operating parameters are mass flow rate of 200–400 ml/min with Reynolds number (Re) 400–1350, heat flux of 10, 20, and 30 W/cm2, and 1% and 2% of mass fraction (concentration of the MPCM in DI) with constant inlet sub-cooling 25°C. From the results, at Re. 780 and heat flux of 20 W/cm2, the Nusselt number (Nu) shows an improved performance of 6.5% with Ti-1 in the straight channel configuration, when compared to plain DI. Further, Nu enhancement is up to 11.5% in the dimple-cavity and 26.9% in the rib-groove microchannel heat sinks under the same conditions. To clarify the effectiveness of MPCMS in three configurations, the measurement of COP is conducted. The results reveal that the heat transfer outweighs the required pumping power in all three configurations. Overall, the Rib-groove microchannel shows enhanced heat transfer of 131.6% than straight channel heat sinks and 47.85% higher than the dimple-cavity channel using Ti-1.

ABSTRACTConvection across circular concentric gaps between cylinders is crucial in industrial applications, including electronic cooling, heat exchangers, and solar collectors. The effect of longitudinal and annular grooves on the outside surface of the inside tube on natural convective heat transmission in enclosure concentric circular annulus is experimentally investigated in the present work. Twin pairs of circular aluminum cylinders with the same radius ratio, length, and surface area were examined. Each one consists of two concentric circular cylinders. To maintain a steady heat flux, the inside cylinder of each pair has an electrical heater connected. The results indicate that the increases in the Nusselt number of around 25%, 43%, 67%, 123%, 142%, 157%, and 172% are seen for groove depths of 0.05 cm, 0.10 cm, 0.15 cm, 0.20 cm, 0.25 cm, 0.3 cm, 0.35 cm, and 0.4 cm. In addition, increasing the depth of the longitudinal groove raises the Nusselt number by roughly 33%, 51%, 79%, 99%, 136%, 153%, 173%, and 184%. The longitudinal grooves with a depth of 4.0 mm enable a 38% increase in free convection over prior research. An annular groove of 4.0 mm depth increases free convection by 36%. Furthermore, a further advancement over earlier research is attributable to the extensive surface contact area of the inner cylinder made possible by longitudinal or annular grooves.

ABSTRACTIn this paper, flow resistance and convective heat transfer of Al-kerosene nanofuels are studied experimentally. Al-kerosene nanofuels with mass fractions of 0.5 , 1 , and 2 g/L are prepared and applied as the flow medium for flow and heat transfer experiment via a heating facility. The experiment results indicate that the addition of aluminum nanoparticles has significant influence on both flow resistance and heat transfer performance. Compared to kerosene experiment, friction coefficient, heat transfer coefficient, and Nusselt number of Al-kerosene nanofuels all increase with different increasing rates. With a mass fraction of 1 g/L, the increase rate of friction coefficient was 11%, while the increase rate of heat transfer coefficient and Nusselt number is 19% and 12%, respectively. In order to evaluate the overall flow and heat transfer performance of Al-kerosene nanofuels, a performance evaluation criteria (PEC) is evaluated, and the present experimental results prove that the addition of aluminum nanoparticles gave a gain to the overall thermal performance of kerosene. The present study is aimed to provide useful references for regenerative cooling improvements.

ABSTRACTThis study investigates the thermohydraulic characteristics of immiscible two-phase downward flow and heat transfer through porous media in a vertical, cylindrical, and homogeneous porous medium numerically and experimentally. The test section is exposed to a constant wall temperature after filled with spherical beads. Numerical solution of the model is achieved by the finite volume method and applied to a single-phase flow model. The numerical results are experimentally validated according to air/water downward flow, spherical beads, a ratio of particle diameter to pipe radius is 0.412, 0.396 porosity, 0.01 ≤ Re ≤ 500, water to air volume ratio range from 0 to ∞, and saturation ratio from 0 to 1. The results show that the average Nu is nearly constant up to Re = 40. At Re > 100, it is recommended to take inertia and friction effects into account by means of Forchheimer–Brinkman’s equation. For single-phase flow (water or air) and two-phase flow mixtures, the local Nu has higher values at the entrance section and decreases as the axial distance increases until it reaches its fully developed value of 4.37 at the end of the thermal entrance length. The comparison between numerical, experimental, and other available previous results shows good agreement and validates the numerical model.

ABSTRACTThe present experimental study reports the flow and heat transfer characteristics of a synthetic jet issuing from a sharp-edged orifice (diverging-shaped orifice). The experiments are carried out for a varied range of opening angles of sharp-edged orifices (θ = 0°, 30°, 60°, 90°, and 120°), Reynolds number (Re = 3243–8143), different jet-to-surface spacings (z/d = 1–16), and for two different values of orifice thicknesses, t = 5 mm (t/d = 0.33) and 10 mm (t/d = 0.66). The hot-wire anemometry is used to study the flow characteristics of synthetic jet, while heat transfer characteristics are studied by using a thermal imaging technique. The time-averaged flow fields associated with sharp-edged orifices reveal that orifices with t = 5 mm and 10 mm exhibit saddle-backed and top-hat velocity profile shapes, respectively. The results show that for a square-edge orifice (θ = 0°), the heat transfer rate decreases with an increase in orifice plate thickness from 5 to 10 mm, while the opposite trend in heat transfer is observed with sharp-edged orifice. The heat transfer rate with a 10 mm thick sharp-edged orifice is higher than the 5 mm thick sharp-edged orifice for all the tested opening angles. Furthermore, the results also show that for sharp-edged orifices, the heat transfer rate increases with the increase in opening angle from θ = 0° to 60°, while it decreases with further increasing from θ = 60° to 120°. The maximum value of average Nusselt number (Nuavg) is obtained for θ = 60° for both the orifice thicknesses (t = 5 and 10 mm), and this effect is found to be more pronounced for t = 10 mm orifice. For sharp-edged orifice (θ = 60°), the maximum enhancement in Nuavg is found to be 12.66% and 23% higher for t = 5 mm and 10 mm, respectively, compared to the equivalent square-edged orifice (θ = 0°). The cause for variation in heat transfer rate with sharp-edged orifices is interpreted due to the effect of flow recirculation and mass flow rate. A correlation has been proposed for Nuavg as a function of different opening angles. The present finding is useful for the optimization of the synthetic jet geometrical parameters for the effective heat transfer rate.

ABSTRACTThis study prepares an experimental examination of thermal and hydraulic analysis of an absorber plate integrated with different arrangements of twisted turbulators, including longitudinal, transversal, and oblique orientations. These arrangements are considered for both the integral and the interrupted turbulators over Reynolds number range from 9174 to 55048. Nu ratio and f ratio range from 1.15–2.21 and 1.73–7.79, respectively. The interrupted turbulators with oblique arrangements with the attach angles of 30° and 60° come in the second and third places, respectively, and the maximum criterion value in the latter cases is 1.29 and 1.21.

ABSTRACTThis research aims to experimentally investigate the thermal performance of iron oxide nanofluids having different particle shapes for multiple impinging jet flow. The heat transfer performance of nanofluids with spherical like, faced nanocube and nanowires shapes prepared by hydrothermal synthesis was investigated experimentally. The weight percentage of the nanofluids examined in this study is 0.2 wt%, and different jet-to-jet spacing (B) and jet-to-plate distance (H) values were tested for each nanofluid. The experiments were carried out for laminar flow conditions, the temperature distribution was obtained through thermocouples, and the Nu number was calculated for each case. In experiments using nanowires, it was observed that the highest Nu numbers were obtained for jet-target/diameter ratio 2 for all cases (jet spaces and Reynolds numbers). Maximum Nu number enhancement was approximately 27.3% compared to pure water for the specified conditions. This value is quite significant since it was obtained with a relatively low concentration of nanofluids. It was observed that the particle size and morphology shape significantly impact nanofluids’ performance in heat transfer.

ABSTRACTThe local heat transfer characteristics of circular multiple air jet impingement on semicircular concave surface were studied experimentally. For constant Reynolds number (Re = 9,000), the effect of jet to concave surface spacing (z/d = 1, 4.2, 6), ratio of curvature (D/d = 4.28, 6, 8.6), jet-to-jet spacing (s/d = 2.4, 4, 5.6), and number of jets were considered. The local heat transfer characteristics were determined by using infrared thermal camera and thin foil method. Lower curvature value ratio (D/d = 4.28) resulted higher average heat transfer coefficient due to broader spread of heat transfer distribution, whereas higher curvature ratio (D/d = 8.6) resulted in lower average heat transfer coefficient due to steeper heat transfer distribution in the stagnation area. At lower D/d, z/d, and s/d with seven jet configuration, the heat transfer coefficients were found effective as compared to other configurations due to broad heat transfer distribution and less interaction between the jets. At given D/d and s/d configuration, the stagnation Nu at θ = 0° do not coincide with jet hole center axis and this shift is more pronounced at higher z/d value. This could be due to additional push before jet impinging on concave surface. The secondary peaks were observed between two neighboring stagnation regions in Nu distribution along the center longitudinal line for lower z/d and higher s/d values. This resulted due to upwash that was obtained from jet interactions related to collisions with the wall jets. The correlations of stagnation line average Nu and overall average Nu were obtained. The coefficient of variance of local heat transfer coefficient for each configuration was determined, and correlation was proposed to analyze the degree of non-uniformity.

ABSTRACTThe inclined impinging jets owing to higher efficiency are widely used to disperse concentrated and transient heat loads. The temperature and heat transfer characteristics of jet cooling vertical hot plate at different inclinations are experimentally investigated. Experiments are performed to investigate the area average heat transfer over a flat surface of 600 × 300 mm2 placed at different location (z) of 3−20 d from the jet. The investigation is employed for Reynolds number based on hydraulic diameter of 11.5 mm ranging from 10,000 to 75,000. The orientation of the jet varies from 15° to 75°. The study aims to investigate the influence of jet orientation at different area locations on the surface from the point of impingement. The effect in stagnation region with different jet tilt is investigated. The area average temperature variation and characteristic temperature contours with increasing surface area and inclination is delineated. The characteristics of heat transfer on the surface’s periphery are studied. Heat transfer increases by 40% at lower tilt angle as the jet is displaced from 20 to 6 d, whereas the maximum average Nu occurs at higher inclination. The available Nusselt numbers in inclined jet are compared with present experimental results. A new empirical correlation is developed for a complete range of z/d, Re and large radial span.

ABSTRACTThe present work focuses on the experimental and large-eddy simulation (LES) investigation of fluid flow and heat transfer due to the impingement of an unconfined turbulent slot jet on a smooth flat plate. Three LES sub-grid scale models are validated with the experimental observations for first time in terms effect of slot widths. A detailed parametric study is conducted considering nozzle to plate spacing ratio from 4 to 12, non-dimensional slot width from 0.015 to 0.035, and Reynolds number range from 4000 to 12000. The results show an appreciable change in the stagnation point and along the wall jet Nusselt numbers with the change in Reynolds number, nozzle-to-plate spacing and non-dimensional slot width. No significant effect of nozzle to plate spacing is observed along the wall jet Nusselt number beyond the streamwise location at 0.2. The local Nusselt number increases with an increase in Reynolds number while it decreases with an increase in nondimensional slot width. Comparison of various LES sub-grid models with RANS (k-ω SST) turbulence model is performed. The results show that turbulent kinetic energy plays a major role in enhancing the heat transfer rate. Dissipation of the turbulent kinetic energy increases with an increase in nozzle to plate spacing and non-dimensional slot width. Further, a correlation is proposed for the stagnation point and wall jet region Nusselt numbers in terms of effect of slots width, Reynolds number and non-dimensional nozzle to plate spacing.

ABSTRACTIn this paper, two special-shaped separated heat pipes with different condensation section structures are designed. The working medium is water, and the filling rate is 65%. The evaporation section is placed in the hot water tank, and the condensation section is equipped with a cold water jacket. The effects of different structures of condensing section on the thermal performance of separated heat pipes are experimentally studied, and the temperature changes of the two separated heat pipes during start-up and stable operation under different heating power are analyzed. The results show that the heat transfer characteristics of the two separated heat pipes are different due to the different structures of the condensation section, and the heat transfer capacity of sample 2 is higher. At the same time, the temperature fluctuation in the condensing section of sample 2 is also quite different from that in the condensing section of sample 1, and the temperature fluctuation in the condensing section of sample 2 is greater. The wall temperature fluctuates periodically in a steady-state, and a fast Fourier transform is used to analyze the temperature fluctuation.

ABSTRACTIn this study, the flow and heat transfer characteristics of microencapsulated phase change material (MPCM) slurry were experimentally investigated using a newly designed helical coil heat transfer device. The conventional helical coil has been structurally modified with passive enhancement features aiming to further promote fluid mixing. Specifically, 360° plastic tubing with or without wire coil inserts was added after each 180° of the main helical loop to enhance fluid mixing and improve the overall thermal performance of the device. Pressure drop and heat transfer experiments with MPCM slurry were conducted under turbulent flow and constant heat flux conditions. A new friction factor and Nusselt number correlations for MPCM slurry in helical coils with reversed loops and wire coil inserts are proposed. Experimental results show that the structural modifications did enhance the heat transfer performance of MPCM slurry. The experimental results revealed that the phase change process of MPCM considerably enhanced the heat transfer rate of MPCM slurry. Furthermore, the use of reversed loops and wire coil inserts led to better fluid mixing within the coil, resulting in improved convective heat transfer of the MPCM slurry.

ABSTRACTAn intermittent spray cooling system is applied to fuel cell vehicle heat dissipation using a plate-fin heat exchanger. The effects of various system parameters on cooling performances are evaluated. Experimental results indicate that the cooling performance can be increased by 20% to 80% at various operating conditions and fixed spray area percentage of 45%. An empirical correlation is constructed to characterize heat transfer enhancement percentage. For the 124 data points examined, 92.73% of the data are within 20% of the error range, and 81.8% are within 15% of the error range. The average absolute deviation is 9.30%.

ABSTRACTThe present study proposes a suitable injection configuration and operating parameters for the film cooling of an aero-engine afterburner, based on experimental and numerical studies. The experimental investigations are carried out for different injection locations for blowing ratios 0.5 and 1, fixed injection angle (45°), and density ratio (1.095). The numerical study is extended for various injection locations with a wide range of operating parameters such as blowing ratios from (0.25–3), density ratios (1.095, 2.5, and 4), and different pressure ratios from 1 to 15, representing the operating conditions of an actual aero-engine afterburner. The present study reveals that the film cooling on the corrugated surface is strongly influenced by the secondary stream injection locations and corrugation amplitude to wavelength ratio. This study also shows that the film cooling behavior on a corrugated surface is inherently different from that a flat surface; the centreline effectiveness plots show improvements even at a higher blowing ratio (i.e. 2). The density ratio also shows a significant impact on film cooling performance, and with an increase in density ratio from 1.095 to 2.5, the centreline effectiveness increases.