International Journal of Thermal Sciences最新文献

The comprehension of heat transfer mechanisms and their profound implications on biological heat transfer is of paramount importance in the advancement of cancer treatments across all types of malignancies. In the present study, the intricate interplay between Pennes' biothermal principles, Maxwell's electromagnetic equations, and heat generation via a one-slot microwave antenna is resolved numerically. By administering magnetite nanoparticles into malignant tumors, an induced field is engendered, ultimately leading to tumor ablation. By manipulating the microwave frequency, the resultant field is assessed to ascertain the optimal therapeutic modality for this dangerous ailment. The investigation incorporates varying volume percentages of nanoparticles, namely 0.1, 0.05, 0.01, and 0.005 percent, yielding tumor necrosis durations of 2.8, 7.3, 34, and 69 s, respectively. Furthermore, the loss of healthy tissue is quantified as 4.8 %, 15.4 %, 65 %, and 139 %, respectively. Consequently, a direct correlation emerges between the percentage of nanoparticles employed and the diminished treatment duration, as well as reduced adverse effects on healthy tissues, leading to improved patient comfort and minimized thermal-induced injury. Additionally, the influence of frequency within the microwave range (0.3–10 GHz) is probed. Accordingly, when the nanoparticles are injected into the tumor, the frequency has no meaningful difference in the treatment result.

Spectral emissivity is one of the most important parameters of metal radiation, with considerable influence on heat-transfer calculations, infrared signatures, pyrometry, and other high-temperature fields. Therefore, a computational model that can accurately calculate the emissivity of metal surfaces is crucial in these fields. In this study, a modified ray-tracing method was proposed to predict the normal spectral emissivity of a rough metal surface. In contrast to other methods involving a complex refractive index, this approach uses the known spectral emissivity of smooth surfaces as an input to avoid using complex refractive index data, which are more difficult to obtain. In addition to the emissivity data of pure Co and advanced high-strength steels samples (DP980) in the literature, emissivity measurements were conducted on GH5188 superalloy with different roughness values to further verify the modified ray-tracing method. A comparison of the measurement and prediction results revealed the high accuracy of the modified ray-tracing method, with the average and maximum errors of 1.92 % and 9.49 %, respectively, which are within the applicable range. As the new method is based on a geometric optics approximation ray-tracing approach, the model is primarily applicable to short-wavelength bands. Additionally, it has been observed in practical use for cases with low roughness. Moreover, some cases with long wavelength bands that exceed the geometric approximation can still yield highly accurate predictive results. When examining the distinction among various processing methods, higher accuracy can be achieved through two-dimensional measurements of the actual surface for prediction. In contrast, simplified methods, such as using one-dimensional cross-sections or surfaces generated based on the surface roughness for the prediction, yield less precise results.

In this study, the heat transfer and flow resistance characteristics of a trifoliate petal twisted helically coiled tube (TPTHCT) were numerically studied. First, a mathematical model of the TPTHCT was established and the enhanced heat transfer mechanism was analyzed. Secondly, the TPTHCT and the smooth helically coiled tube (SHCT) were comparatively investigated mainly on the parameters of heat transfer coefficient (h), pressure drop (|Δp|), heat transfer effectiveness (ε), and heat transfer exergy loss number (ξ_HT). Finally, a Multi-Objective Genetic Algorithm (MOGA) was utilized to optimize the TPTHCT. The results indicate that an additional torsional force exists in the TPTHCT, which causes a more complex flow state and makes the temperature distribution more uniform. For the same working condition, h increased (by a maximum of 40 %), |Δp| increased (by a minimum of 50 %). ξ_HT consistently exhibited an opposite variation trend with ε at the same inlet temperatures ratio (τ); further ξ_HT can describe the effect of changes in τ, whereas ε cannot. The best result from the optimization was achieved with the objective function being maximum h, minimum |Δp|, and minimum ξ_HT; and the optimal structural parameters were R_co = 25.1 mm, P = 93.2 mm, d_i = 8.8 mm, r₂ = 0.3, θ = 432°, a = 0.16, E = 151 mm.

Different ignition time leads to different spread and prior evaporation processes of the leaked liquid fuels before being ignited. Under the coupling effect of the kinetic characteristics of the spread fuels in inclined substrates and the heat transfer mechanisms to the fuel layer from fire plumes, the spill fire behavior with different ignition time on inclined substrates in confined space is more complex. The relevant experimental tests and theoretical analysis can provide significant theoretical and technical support for the risk control of spill fire. Tests were conducted with ethanol at a spill rate of 39 ml/min on inclined steel trenches with different slopes of 0°, 1°, 3°, 5°, with instantaneous ignition and different delayed ignition time such as 10 s, 20 s, 30 s. Based on the thermocouple test data and MATLAB image processing, parameters such as burning area, spread rate, burning rate and flame height were analysed. The maximum burning area is increased with increasing delayed ignition time for spill fires on the same sloped substrate. The quasi-steady burning area is independent of the delayed ignition time, but increases with the increasing of the substrate slope. Considering the different absorption of the radiative heat feedback by the fuel layer in confined space, a modification is proposed in the existing burning rate model of spill fires. It was found that the spread rate of spill fire increases with increasing slope, which increases along with the increasing delayed ignition time on the substrate of the same slope. By integrating the effects of burning rate, burning radius, combustion heat of the fuel, and the inclination angle, etc., an equation for fitting the flame height is derived. It is found that the flame height in the quasi-steady burning phase decreases with increasing substrate slope and is independent of the delayed ignition time.

Aiming to enhance the overall cooling characteristic for high heat flux density and high input power operating chips, the jet nozzle diameters and spacing distribution of the jet nozzle were parametrically investigated. The findings demonstrate that the heat dissipation characteristics of the heating surface presented a volcanic change trend and the heat sink with 2 mm jet nozzle diameter creates a visible tradeoff for thermal and hydraulic characteristics. The larger or smaller jet nozzle holes can weaken cooling capacity. Furthermore, the results of the comprehensive evaluation index PPTR also demonstrated that the overall cooling characteristics of the 2 mm jet nozzle holes were optimal. The distribution pattern of jet nozzle holes on the diverter orifice plate plays an important role in balancing between thermal performance and power consumption. The results show that the non-uniform jet holes have been greatly improved comparing the uniform distribution of jet nozzle holes, and the temperature uniform performance was maximum increased up to 54 %, the power consumption was a maximum decrease of about 9.6 %. It was concluded that the change of the spacing distance and diameters for the jet nozzle has an important value for improving the heat dissipation function of heat sinks.

The threat of icing caused by supercooled water droplets on aircraft components such as wings and compressors is a serious concern for aviation safety, and the surface roughness of these components can experience alterations due to environmental corrosion or damage. However, the potential impact of surface roughness variation on their susceptibility to icing remains unclear. In this study, the freezing experiments of sessile water droplets on the surfaces of a typical aero-engine titanium alloy (ZTC4) with varying roughness were conducted using a laser confocal micro-Raman spectrometer with a heating/freezing stage. Three freezing stages were captured via an optical microscope, and the temperature changes of water droplet during the cooling process were explored through heat transfer simulation. In addition, the relationship between Raman peaks and temperatures of frozen droplets was quantified. The results of 200 repeated experiments demonstrated that the freezing temperatures of sessile water droplets exhibited a two-parameter Weibull distribution, and there was a nonlinear positive correlation between the mean freezing temperature and surface roughness, which implied that environmental corrosion or damage leading to an increase in surface roughness may significantly elevate the probability of component icing and safety issues.

Aerodynamic heating and impact resistance present significant challenges for hypersonic aircraft. This study introduces a novel multi-jet strategy to enhance both resistance reduction and thermal protection performance of hypersonic aircraft. Computational Fluid Dynamics (CFD) analysis is employed for the aerodynamic evaluation. The results demonstrate that this novel strategy effectively mitigates the shock waves, the peak pressure coefficient and Stanton number have been reduced by 64.3 % and 73.2 %, respectively. Through a comprehensive analysis of the influencing factors, it has been found that increasing the pressure ratio of the root jet significantly lowers the heat flux and pressure on the blunt body, albeit at the cost of an increased total flight resistance. When the nozzle on the side of the airway is oriented perpendicularly to the incoming flow direction, a notable reduction in resistance and aerodynamic heating on the blunt body is noted. By increasing the length-diameter ratio of the spike, a significant decrease in the pressure coefficient of the blunt body is achieved, the Stanton number remains largely unaffected. This study offers insights into the engineering application of strategies for reducing resistance and heat in hypersonic aircraft.

In restricted space, fuel combustion is often constrained by sidewalls and corners, which can alter the air entrainment compared to unrestricted scenarios. To investigate the impact of sidewalls and corners, this paper presents numerical simulations of rectangular propane gas fires under sidewall and corner restricted conditions. The burner had an area of 300 cm², with 5 different aspect ratios (n = 1, 1.5, 2, 3, and 4) and 4 heat release rates of the fire source (12 kW, 18 kW, 24 kW and 30 kW). The results demonstrate variations in the distribution of the flow field under different restricted conditions. For wall fires, when the long side of the burner is against the wall, the velocity in the X direction (u) approaches 0, and the lateral flow velocity on the X–Y plane (V) and the velocity in the Y direction (v) are nearly identical near the short side of the burner. Velocity V near the burner side can be divided into three zones: the high-velocity zone, the low-velocity zone, and the locally high-velocity zone. For corner fires, V increases as it gets closer to the wall. With the increase of n, the streamline where the velocities u and v are equal moves to the long side of the burner. Additionally, the fire entrainment rates of rectangular wall and corner fires at different heights from the burner surface were quantitatively studied. On the basis of the fire entrainment model for square and circular fires in open space, by introducing the equivalent diameter $D_{\text{eff}}^{*}$, a modified entrainment rate model at different heights from the burner surface is proposed, which can well predict the entrainment rate of rectangular wall or corner fires when the aspect ratio of the rectangular burner ranges from 1 to 4 and ${\dot{Q}}_{\text{eff}}^{*}$ ranges from 0.97 to 6.79.

Flow boiling in microchannels is a promising approach to solving the heat dissipation challenge in high-power microelectronic components. However, its critical heat flux (CHF) has always been limited by local dry-out and chaotic two-phase flow. Thus, to overcome the dilemma and increase the heat dissipation capacity as much as possible, this study presents the concepts of short counter-flow microchannels (SCM). On this basis, four short counter-flow microchannels with different numbers of interconnected slots (SCSM) are further fabricated. Flow boiling experiments on deionized water with mass fluxes of 118–219 kg/m²·s are carried out in SCM and SCSM, with comparisons made to traditional parallel-flow microchannels (PM). Visualization studies elucidate the flow boiling enhancement mechanisms, and the influence of slot numbers on the flow boiling process is investigated. Results indicate that the CHF of SCM is enhanced by 160.6%–204.4 % compared with PM. Moreover, as the mutual replenishment among microchannels is realized in SCSM, the CHF can be further improved by 181.7%–278.2 %. Given the promotion of nucleate boiling and redevelopment of the boundary layer, the range-weighted average heat transfer coefficients (HTC) of SCM and SCSM are improved by 56.7%–98.2 % and 54.9%–263.4 % compared to PM. As the shift in boiling characteristics, the enhancement mechanism of SCSM on HTC in high and low heat fluxes is different. It is worth noting that the enhancement ratio of both CHF and HTC increases with the slots number in SCSM. Meanwhile, interconnected giant bubbles formed in SCSM through the slots are unstable and easily fractured, particularly in SCSM with more slots. The enhancement in heat transfer performance has not come at the cost of increased pressure drops, which are reduced by 75.9%–80.4 % and 77.2%–86.6 % in SCM and SCSM, respectively, compared with PM. More importantly, the additional expansion space alleviates the reverse flow of the bubbles, and therefore the noticeable suppression of boiling instability is achieved in SCSM. A highly efficient and stable flow boiling process is obtained in this work.

A double-circle straight-channel printed circuit heat exchanger (PCHE) was first employed in a synergistic air-breathing rocket engine (SABER) system for supercritical He and H₂ heat transfer. The heat transfer mechanism of the supercritical flow in the PCHE channel was numerically analyzed. For considered the pressure drop, heat transfer efficiency, and ratio of heat flux to weight, simultaneously, the design of the PCHE is multi-objective genetic optimized. The results shown that the supercritical H₂ flow is chaotic near the pseudo-critical point, which is a coupled effect of buoyancy and gravity. Chaotic flow leads to an asymmetrical temperature distribution, which deteriorates the heat transfer performance. For 27 numerical experimental cases designed using the center composite surface method, the determine factors of the regression models of the three objectives for both cold and hot sides were all above 92 %. The Pareto optimal solutions for the supercritical He - H₂ PCHE design and performance were obtained based on the nondominated sorting genetic algorithm II. From a comprehensive view of the three targets, the optimal designs were the A-4 and B-4 solutions for the cold and hot sides, respectively.