Journal of Engineering Thermophysics最新文献

Entropy and exergy analysis of a thermal system are the most powerful tools that can be employed to specify the optimum operating conditions of that system and utilization rate from the system. In the experimental thermal system in this work, entropy generation and exergy analyzes of GO–DW nanofluids have been carried out in straight copper pipes with constant heat load and 8 mm and 16 mm inner diameters. While the heat loads applied to the pipes are 250 W and 350 W, the range of fluid flow rate values in the pipes is 0.9 l/min–1.8 l/min. GO–DW nanofluids with 0.01% and 0.02% volumetric concentrations and DW have been used as working fluids in the pipes. The outcomes acquired from this work have been matched with the studies using different nanofluids in the literature and it has been noticed that the outcomes are reasonable and consistent. The results of the study have been presented at different GO–DW nanofluid concentrations in pipes with 8 mm and 16 mm inner diameters as thermal, friction and total entropy production, output exergy ratio and 2nd law efficiency. The obtained outcomes have exhibited that the lowest total entropy generation has been obtained for the 8 mm diameter pipe and the nanofluid with 0.02% GO–DW concentration. Besides, 2nd law efficiency is 12% higher for the 8 mm diameter pipe than 16 mm at flow rate of 1.2 l/min and 0.02% GO–DW nanofluid, and 350 W heat load.

This experimental study presents the thermal efficiency enhancement of a parabolic trough solar collector (PTSC) system using different refractive surfaces and various mass flow rates. Two PTSC models were used to compare the aluminium sheet (AS) and silver chrome film (SCF) under the weather conditions of Hungary. Initially, similarity tests of the two collectors were carried out using the aluminium reflective surfaces with a mass flow rate of 90 L/h. According to the test results, the average thermal efficiency between collectors did not exceed 0.3%. Afterwards, the PTSC was compared with an evacuated U-shaped glass tube at different mass flow rates, namely 30, 60, 90, and 120 L/h. According to the experimental results, the maximum heat removal factor of PTSC for both SCF and AS at 120 L/h was 58.59% and 46.02%, respectively. Moreover, the maximum thermal efficiency with AS obtained for 120, 90, 60, and 30 L/h mass flow rates reached 27%, 22.84%, 18.9%, and 14.86%, respectively. Likewise, the maximum thermal efficiency with SCF at these mass flow rates attained 46.84%, 39.73%, 37.47%, and 33.68%, respectively. Conclusively, the PTSC thermal performance using SCF is superior to that of AS regardless of mass flow rate.

Recent experimental studies revealed the development of slip at the interface of a steady axisymmetric swirling flow of two immiscible fluids. As swirl increases, the slip changes the flow topology scenario compared with that numerically predicted using the velocity continuity condition. This phenomenon of fundamental and practical interest has not been well understood yet. What condition should replace the velocity continuity has remained unknown. Our study addresses this problem by providing detailed experimental and numerical investigations of the flow in the interface vicinity. The bulk flow is driven by the rotating lid in a vertical cylindrical container—a model of vortex bioreactor. The centrifugal force pushes the upper fluid from the axis to the sidewall near the lid and the fluid goes back to the axis near the interface. This centrifugal circulation drives the anti-centrifugal circulation of the lower fluid at a slow rotation. As the rotation speeds up, a new flow cell emerges below the interface-axis intersection. It expands radially and downward occupying the lower-fluid domain. During these topological transformations, the flow remains steady and axisymmetric with no visible deformation of the interface. Using the advanced PIV experimental and numerical techniques, we explore the distribution of azimuthal and radial velocities in the interface vicinity and reveal that the azimuthal velocity is continuous while the radial velocity has a jump at the interface. The radial velocity tends to zero in the upper fluid. In contrast, the radial velocity does not tend to zero and satisfies the stress-free condition in the lower fluid at the interface. The numerical simulations under these conditions are in qualitative agreement with the experiment.

A mathematical model that incorporates thermal radiation, viscous dissipation, heat source/sink, chemical reaction, and suction was used to study the MHD flow of Casson nanofluid over a nonlinearly permeable stretched sheet. The governing partial differential equations are composed of a set of nonlinear ordinary differential equations using proper similarity transformations, and then solved using the homotopy analysis approach (HAM). Numerical data and plots are used to discuss the impact of physical limitations on liquid velocity, temperature, and concentration. To examine the flow characteristics at the wall, the skin friction coefficients, local Nusselt number, and Sherwood numbers are also evaluated. With much acclaim, a link between penetrable findings for specific cases is discovered.

The results of studies on reducing ice adhesion via change in the original shape of the blade profile and its surface by means of microstructuring of different geometries are summarized. The influence of the height of nanograss on the intensity of ice formation was estimated for coatings investigated under icing conditions on a climatic aerodynamic stand for study of icing on model blades of wind turbines. The prospects and limitations of using polymeric plastic coatings for anti-icing systems for wind turbine blades are shown.

In this work, experimental data on the heat transfer in a horizontal layer of liquid are obtained for wide ranges of the layer height and reduced pressure. Explosive boiling-up of liquid at low reduced pressures, when significant fluctuations of the pressure, heating surface temperature, and heat flux were observed, is considered. For this condition, the problem of determination of the temperature head is analyzed, as well as uncertainties of its measurement. The experimental data obtained at different layer heights and pressures were compared with the known calculation formulas derived for determination of the heat transfer coefficient during nucleate boiling for analysis of the range of applicability of these formulas. It is shown that at low reduced pressures, the experimental data are generalized well by Yagov’s formula, and the experimental data obtained at moderate reduced pressures at nucleate boiling are generalized well by Gogonin’s formula. Both formulas were obtained for pool boiling. With the help of Pioro’s formula in which we corrected the coefficients and exponents at the Prandtl number and heat flux, we managed to summarize the experimental data over the entire range of the reduced pressure and the liquid layer height.

Numerical investigation of heat transfer augmentation with Al₂O₃ nanofluids-based crude oil in a shell and tube heat exchanger. This paper presents numerical and experimental investigations to study the effect of using Al₂O₃ nanofluids based crude oil on heat transfer enhancement in a turbulent regime with mass flow rate of (4 to 18 kg/s) in the shell and tube heat exchanger. The investigation concentrates on the effects of the Al₂O₃ based crude oil nanofluids on friction factor, flow characteristics and heat transfer, through shell and tube heat exchanger. The results show that the thermal conductivity as well as the viscosity of Al₂O₃ nanofluid based crude oil increased with increasing nanoparticles volume fraction and decreased with increasing the temperature. The outcomes revealed that the Nusselt number improved with increasing mass flow rate and also the friction factor increases dramatically using nanofluid this because of increment in nanofluid viscosity comparing to the base fluid (crude oil), additionally the results illustrate a constant pattern along the heat exchanger.

Laser shock hardening consists in the action on a sample of a pressure pulse generated by the plasma of the sample surface layer vaporized by short high-energy laser pulses. To model the change in the properties of the material under the action of pulsed mechanical stresses, it is necessary to know the space-time dependence of the pressure acting on the sample subject to the parameters of the laser pulse and the properties of the evaporated material. This paper presents a model and calculation of the time dependence of the pressure acting on a sample as a result of evaporation of the absorbing material and the development of plasma optical breakdown.

It is shown that it is necessary for vacuum plasmatron with hollow cathode to meet the technical requirements to the hollow cathode pipeline to provide not only the necessary kinetic energy of the gas involved in the formation of working parameters in the cavity cathode but also to ensure the stable operation conditions for vacuum plasmatron at large current without the occurrence of high-frequency oscillations in the plasmatron electrical circuit. The pipeline maximum length has been established, guaranteeing the speed of gas at its final section and equals to the speed of sound at the output; the results of mathematical modeling and experimental investigated parameters for developing gas-dynamic processes in hollow cold and hot cathodes of vacuum plasmatrons are presented. The start-up modes ranges for warming up the cavity cathode and continuous discharge output with hollow cathode into working modes with flowing currents up to 10000 A are considered. The occurrence and development of the gradient pressure, density, velocity mass flow rate at heating the cathode and the gradient increase temperature effect of the cathode edge with forming current conductivity active zone in the cylindrical cathode are shown.

The heat transfer during boiling of freon R-21 in vertical downflow for an element of a plate-fin heat exchanger with textured and perforated fins was studied experimentally. The experiments were carried out for mass velocities of 30 to 60 kg/m²s and heat fluxes of 900 to 1900 W/m² for a heat exchanger with 850 fins per meter. The heat exchanger fin texture at an angle of 45 degrees to the flow direction of the vapor-liquid mixture made it possible to intensify the heat transfer and suppress the deterioration of the heat transfer at a vapor quality exceeding 0.8 in comparison with perforated fins. It has been found that the heat transfer coefficient is practically independent of the heat flux density and mass velocity, which shows the decisive influence of evaporation on the liquid film surface on the heat transfer under these conditions.