This study examined the effect of surface roughness on the pool boiling heat transfer coefficient of pure water and water-alumina nanofluid with 0.1% and 0.01% volume concentration using computational fluid dynamics on the surface of a stainless-steel cylinder. The effect of nanoparticles was checked by averaging the thermophysical properties in the equations of the flow field with boiling. Simulations were performed for initial surface roughnesses from 0.1 to 0.8 µm. Furthermore, the presence of nanoparticles would make their deposition on the heated surface and change the surface properties. Thus, once again simulations were performed for roughness with the same values but because of the deposition of nanoparticles. In doing so, two separate equations were used for the nucleation site density parameter. Ultimately, the results obtained from both types of roughness were compared. The results indicated that with an increase in the roughness, the boiling heat transfer coefficient increased. Further, at the same roughness, the boiling heat transfer rate of the deposited surface decreased for nanofluid of 0.01% vol and increased for nanofluid of 0.1% vol compared to the non-deposited surface. For pure water at 0.8 µm roughness, the sediment improved heat transfer but it reduced heat transfer for 0.4 µm and lower roughness.
Forced heat transfer of Al2O3-water, CuO-water, and TiO2-water nanofluids, Al2O3-CuO-water, Al2O3-TiO2-water, and CuO-TiO2-water nanoparticles in volumetric concentrations of 0.5%, 0.9%, and 2% were studied in a flat tube car radiator. In this work, the thermal performance of hybrid nanofluid will be compared with mono nanofluid and pure water base fluid. These nanoparticles were combined in ratios of (75:25), (50:50), and (25:75) in pure water. Mono and hybrid nanofluids with an inlet temperature of 90 °C, and in different Reynolds numbers (272–816) were studied and numerically simulated. The heat transfer performances of mono and hybrid nanofluids were determined using Nusselt number (Nu), overall heat transfer coefficient ( U), convective heat transfer coefficient ( h), and heat transfer rate ( Qa). The results show an enhancement in the thermal performances of the radiator with an increase in Reynolds number and volume concentration as follows: (Al2O3-water > Al2O3-CuO-water (75:25) > Al2O3-CuO-water (50:50) >Al2O3-CuO-water (25:75) > Al2O3-TiO2-water (75:25) > CuO-TiO2-water (75:25) > Al2O3-TiO2-water (50:50) > CuO-TiO2-water (50:50) > CuO-water > Al2O3-TiO2-water (25:75) > CuO-TiO2-water (25:75) > TiO2-water).
We present the detection and analysis of the full-orbit phase curve and secondary eclipse of the short-period transiting hot Jupiter system WASP-19b with a single joint fit to photometric data and resolve parameter degeneracy. We analyze data taken by the Transiting Exoplanet Survey Satellite (TESS) during sectors 9 and 36. We model the data with our five-component model: primary transit, secondary eclipse, ellipsoidal variations, thermal emission, and reflected light, which are jointly fit to extract the information from all parameters simultaneously. The amplitude of Doppler beaming was also estimated to be ∼3 parts-per-million (ppm), but given the precision of the photometric data, we found that it was negligible and excluded it from the total phase curve model. We confidently report the secondary eclipse depth of 494 − 48 + 59 ppm, the most accurate eclipse depth determined so far for WASP-19b, after cleaning the data from the instrumental systematic noise. According to the TESS bandpass, the day and nightside temperatures of WASP-19b are 2245 − 20 + 19 K and 1095 − 21 + 20 , respectively. In addition, we find that the region of maximum brightness is well aligned with the substellar point, implying that there is an inefficient heat distribution from the dayside to the nightside. Our derived A g = 0.11 − 0.03 + 0.03 , suggesting that WASP-19b’s geometric albedo is greater than the geometric albedos of most other hot Jupiters. Finally, the ellipsoidal variation signal amplitude we calculated agrees with theoretical expectations. Our comprehensive model with the approach of Markov Chain Monte Carlo shows the remedy of the degeneracy parameter in photometric data.
In this research, a theoretical model is presented to investigate the density wave oscillations (DWOs), in two horizontal parallel channels with lumped parameter model based on two phase homogeneous hypothesis. The parallel channel is composed of the entrance section, heating section and outlet section and the model consists of the boiling channel model, pressure drop model, parallel channel model, constructive model and inertia and compressibility effects, while subcooled boiling effect is neglected and the governing equations are solved by Gear method. The model is validated with experimental data of a single channel flow instability experiment. Then the flow instability in twin channel system is studied under different conditions. This model can analyze the effects of external parameters, such as fluid inertia and compressible gases on the stability margins of density wave oscillations. The results show that, the fluid inertia and compressible gases can significantly change the stability margins of two parallel channels; in fact, the stability behavior of two parallel channel system improves with increasing the inlet inertia and outlet compressibility but, increasing the outlet inertia and inlet compressibility have negative effects the system stability.
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