Heat and Mass Transfer最新文献

Current research is investigating the effect of different tube geometries on flow patterns and thermal performance. Perform numerical simulations and thermo-fluid couplings. Calculation results are calculated and the solution uses both flow transport and thermal correction. Results are compared and validated using experimental results. Hydraulic and heat flow behaviors in all corrugated tubes are studied and discussed under various constitutive parameters of position and shape. The turbulent fluid flow in these tubes is modeled using 3D numerical flow domain simulations and the optimization of the multilens algorithm is analyzed. The effects of various geometric design parameters such as ring diameter, spacing between each well ring, and number of well rings around the tubing spacing of the rings were analyzed using Response Surface Methodology (RSM) and Taguchi Method (TM). Analyzed. be studied. The effects of changes in flow structure, such as velocity magnitude and radial velocity, and velocity magnitude and radial velocity profiles in different configurations, are studied. An experimental design strategy using the Taguchi method (TM) is chosen according to the variance of the orthogonal L16 sequences. Optimization results show that higher differential pressure values are related to shaft diameter. Therefore, the number of corrugated rings has a great effect on the heat transfer rate and temperature difference. Various configurations of Conduit Performance Evaluation Factor (PEF) increased the PEF value by more than 1.3.

This study investigates the comparative analysis performance of three environmentally friendly refrigerants, R1270, R290, and R600a, in the context of flow boiling heat transfer (FBHT) and pressure gradients. The experiments employ copper microfin and smooth tubes, operating under varying conditions, including saturation temperatures (T_sat) of 6 and 15 °C, heat fluxes (HF) ranging from 13 to 30 kW.m⁻², mass fluxes (MF) spanning 187 to 427 kg.m⁻².s⁻¹, and vapor quality from 0.1 to 1.0. Both tube types share identical dimensions - an outer diameter, inner diameter, and length of 7 mm, 6.14 mm, and 500 mm, respectively - facilitating a focused investigation into the impact of microfins on flow boiling characteristics. The results highlight noteworthy differences among the refrigerants, with the microfin tube exhibiting substantial enhancements in heat transfer coefficient (HTC), particularly pronounced with R1270 and R290. At the same time, the R600a demonstrates more HTC improvements than the smooth tube. Additionally, the microfin tube increases pressure gradients. The average enhancement factor (EF) for R600a, R290, and R1270 are 2.15, 1.95, and 1.9, respectively, while the average penalty factor (PF) for R600a, R290, and R1270 are 1.25, 1.3, and 1.35, respectively. Comparative analyses with established literature correlations validate the experimental findings.

Geometry optimization aims to maximize heat transfer rate and minimize pressure drop attending to structural and fabrication constraints. The present work carried out the first optimization study of compact heat exchangers produced by selective laser melting (SLM) with circular channels. No optimization study investigated circular mini channels since most focus on semi-circular channels of printed circuit heat exchangers. Besides, since samples produced by selective laser melting present higher yield strength, it was possible to investigate a higher range of configurations. Analytical models of heat transfer and pressure drop, with structural analysis in finite element model were used in the optimization study. The analysis was conducted using genetic algorithms (NSGA-II) based on evolutionary and dominance concepts to evaluate different configurations. The results showed a strong relationship with the admissible stress limit, so a new study, using the properties SLM samples, was performed. Decision variables’ behavior was investigated among all the optimum solutions, besides stress constraint and flow type (cross and counter-flow), resulting in different optimal solutions of Pareto curves. The optimization provided heat transfer and pressure drop ratio from 1.2 kW/Pa to 12.5 kW/Pa. The optimized arrangements were compared with heat exchangers from the literature, demonstrating a 19% improvement in thermal performance and an 85% reduction in pressure drop.

In the traditional tobacco drying process, there is often a problem of uneven drying, which is closely related to drying conditions such as air velocity and temperature. To better understand the drying characteristics of tobacco, its drying kinetic performance were experimentally studied and predicted in this paper. In the drying experiment, the range of air temperature and velocity is 20–60℃ and 0.95–4.93 m/s, respectively. The results show that the effective diffusion coefficient increases with the increase of air temperature and decreases with the increase of air velocity. The effective moisture diffusivity(({D}_{eff})) ranges from 2.077 × 10^–7 to 9.136 × 10^–7 m²/s. Additionally, the activation energy (E_a) is between 14.292 and 21.032 kJ/mol according to Arrhenius law. Among the six commonly used empirical correlations, the logarithmic model has higher prediction accuracy, but it has a prediction deviation of more than 20% in the later stage of drying. Based on the logarithmic model and the two models, a new prediction model of tobacco drying characteristics was proposed with a maximum relative deviation error of less than 1%.

The direct contact evaporation of n-pentane volatile liquid drop in a warm flowing immiscible liquid (water) has been investigated experimentally. A Perspex column with a 10 cm internal diameter and 100 cm active height was used in the experiments. N-pentane at its saturated temperature (~36 °C) and distilled warm water were utilised as a continuous and dispersed phase. The warm water, with three different Jacobs numbers (Ja), (Ja = 6.1, 23 and 46.3), flows from the top of the column and leaves from the bottom at three different Reynolds numbers (Re = 3250, 6500 and 9750). The evaporation of the drop while rising along the column was filmed with a Photron FASTCAM high-speed camera ((sim)65,000 f/s). All images were analysed using AutoCAD, and the two-phase bubble, the vaporisation ratio (left(xright)) and the half-opening vapour angle (left(beta right)) were measured. The convective heat transfer coefficient in terms of Nusselt number (Nu) was predicted based on the measured two-phase bubble radius through the experiments. The effect of Reynolds’s number (Re), Jacobs’s number (Ja), vaporisation ratio (x), and diameter ratio (B) on Nu were investigated. The experimental results revealed that Nu increased with time. The Re and Ja significantly affected the time-dependent Nu. Although the final Nu was nearly the same for all cases (Nu = 21), the higher the continuous phase Re, the higher the Nu, especially with the progress of evaporation (left(tau ge 70right)). In addition, the results showed that Ja inversely influenced the average Nu, and the final value of Nu depended strongly on Ja. The higher the Ja, the lower the average Nu and the shorter the time for complete evaporation. In this regard, the dimensionless time (left(tau right))required for complete drop evaporation was about 38, 60 and 120 for Ja of 46.3, 23 and 6.1, respectively.

The shell-and-tube heat exchanger (STHX) is probably the most ubiquitous form of heat exchanger in many industrial settings. In the current investigation, various nanofluids (Al₂O_3, PCM, CNT, Al₂O₃+PCM, and Al₂O₃+CNT) at volume concentrations of 0.01% and 0.1% were used to test the hydrothermal performance in a STHX. This study aimed to investigate the influence of the Nusselt number and friction factor on the Reynolds number and the hydrothermal performance of STHX at various volume fractions. The execution of an experimental investigation accomplished this. The findings demonstrated that the pressure drop and heat transfer coefficient depend on the nanofluid's flow rate, that it is superior to DI water and improves with volume. The h_i/∆p value rises for Al₂O₃ due to pressure drop impacting heat transmission, but it falls for phase change material (PCM) and Al₂O₃+PCM nanofluids. The hybrid nanofluid Al₂O₃+CNT flowing at 10lpm in the tube has a 15.60% greater friction factor and an average Nusselt number of 38.08% compared to the base fluid. The heat transfer coefficient, Nusselt number, pressure drop, and friction factor for Al₂O₃+PCM at 8.33lpm increase by 9.18%, 8.91%, 36.84%, and 5.98%, respectively, with an increase in volume concentration from 0.01 to 0.1%. Nanofluids that are either mono- or hybrid and contain PCM dispersion have a better heat transfer coefficient at low flow rates. The pressure loss increases with increasing flow rate because PCM particles raise dynamic viscosity.

This study investigates the effect of gravity on the flow pattern and thermal efficiency of a single-loop oscillatory heat pipe. To simulate the influence of gravity, the deployment angles of the mechanism are varied (30°, 45°, 60°, and 90°). OpenFoam software is implemented to model boiling and condensation in the oscillating heat pipe, utilizing the volume of fluid (VOF) method. The evaporator is supplied with 55.5 W of heat power, the condenser wall temperature is maintained at 300 K, and the filling ratio of heat transfer fluid (water) is 40%. The findings revealed that decrease in gravitational force results in the thermal resistance be increased and the thermal performance of heat pipes be diminished. Expectedly, the best thermal performance in the oscillating heat pipe is observed in vertical mode, however, this study also examines the influence of reduced gravity. The simulation results show that the bubble pattern is first initiated by the bubble nucleation at the start of the heating process. Consequently, by bubble coalescence the slug and annular regimes can be observed. The phenomenological analysis of the dissolution, bubble coalescence, growth, and contraction observed in this study are discussed.

The current study aims to improve the thermal and energetic performances of building materials used in construction field and especially to meet the heating and cooling needs required by the Moroccan thermal building regulations (RTCM 2015). The study aims to investigate the possibility of incorporating wood ashes or crushed waste from traditional pottery into the formulation of eco-friendly bricks. Laboratory-scale experiments were conducted on different mixtures to determine the optimal dosage that would result in optimal thermal characteristics for the brick blocks. The percentage of wood ashes and crushed pottery waste was varied from 0 to 50% relative to the total mass of the dry mixture. Samples of the clay were used to create brick blocks and cylindrical specimens with dimensions of 5 cm in diameter and 10 cm in height. The optimal dosage of wood ashes was found to be 5% in combination with the clay. The addition of crushed pottery waste improved the absorption of these blocks, and the highest thermal resistance values were recorded with a dosage of 20% pottery waste. By replacing 5% of the clay with wood ashes or 20% with crushed pottery waste, it was possible to produce eco-friendly blocks with an increase in thermal resistance comparable to that of traditional building materials. In addition, simulations study of the dynamic thermal behaviour of a room with a single thermal zone was investigated in order to determine the effect of the introduced building material on heating and cooling loads in Morocco. Two climate zones are considered: Agadir recognized by its humid climate (zone 1) and Marrakech city recognized by its dry climate (zone 5). The study aims to compare numerically the heating and cooling demands of hollow brick, hollow block as a common construction material and a comparison with using the introduced material as an energy-efficient material. The results found show that the studied composite material meets the requirements of thermal regulation in building of Morocco (RTCM2015).

The direct emission of coal mine methane caused serious environmental pollution and resource waste. To improve the methane utilization, the porous media burner was regarded as an efficient method. In this paper, the bluff body was proposed to combine with porous media to optimize the combustion characteristics. The effects of bluff body position, size and shape on the temperature distribution and gas emission were studied at different operating conditions. The results indicated that the position of the bluff body greatly influenced the combustion characteristics, and the maximum temperature of 1295 K was obtained at the position of 62 mm. The increase of the bluff body diameter promoted the flame moving to the burner outlet. And the combustion temperature increased first and then decreased when the bluff body height increased. Moreover, the CO and NO_x emissions at the height of 20 mm reached 31 and 16.8 ppm respectively. The combustion temperature was significantly improved by increasing the equivalence ratio and velocity. Compared with the single porous media, the addition of the bluff body increased the combustion temperature and reduced the CO emission by 11%.

Previous studies have demonstrated that altering the surface structure can enhance heat transfer. In this study, a square micropillar array with a homogeneous structure was designed for a long rectangular channel with a hydrodynamic diameter of 10 mm. Deionized water and HFE-7100 were used as working fluids for the study. The effect of flow rate and subcooling degree on flow boiling heat transfer performance is discussed. The bubble behavior of two different media was compared by visualization experiments. The results show that the square microcolumn array will delay the ONB point by increasing the heat transfer area and disturbing the main fluid, and improve the overall boiling heat transfer performance by 2–3 times. It was found that HFE-7100 boils better under low heat flow density, but its stable nuclear boiling time is shorter. Furthermore, the effects of volume flow and subcooling on heat transfer performance vary significantly at different stages of the boiling process. Before the ONB point, an increase in volume flow will increase the heat current density by 88.9% and reduce the boiling heat transfer stability. After the ONB point, the effect of fluid flow on the boiling process weakens.