International Journal of Heat and Fluid Flow最新文献

The drastic variations in thermal properties of supercritical CO₂ near its pseudo-critical point induce the formation of complex boundary layer structures within pipelines, rendering it susceptible to heat transfer deterioration under the influence of buoyancy forces. The Helmholtz self-excited cavity can generate self-excited pulsating jets, enhancing the intermixing between fluids inside the heat exchanger tubes, disrupting the thermal boundary layer, and suppressing heat transfer deterioration. In this study, the characteristics and mechanisms of self-excited oscillations of supercritical CO₂ flowing vertically upward in the Helmholtz self-excited cavity were investigated using the large eddy simulation (LES) method. A detailed analysis of cavitation and vortex evolution within the cavity was conducted, along with an exploration of the influence of inlet pressure and structural parameters on the frequency characteristics of pulsations. The results indicate a close relationship between cavitation and vortex interactions and the pulsation frequency. An increase in inlet pressure leads to a significant cavitation phenomenon near the jet shear layer and an increase in vortex frequency. Dimensionless cavity length (L_c/d₁) enlargement results in an increase in outlet pulsation frequency but a decrease in pulsation amplitude. The critical dimensionless ratio of cavity diameter (D_c/d₁) plays a crucial role in maintaining the desired pulsation frequency and amplitude. Within the working range outlined in this paper, practical insights for system design and operation are provided by the optimal parameters of L_c/d₁ = 3 and D_c/d₁ = 10.

A combined groove and truss structure is designed for rectangular microchannel heat sinks with 4 % Al₂O₃ nanofluid as the working fluid to address the heat dissipation requirements of high heat flux density electronic devices. The effects of fan shape, elliptical, waterdrop, rectangular, trapezoidal and triangular grooves on the heat transfer characteristics and mechanical properties of microchannels are investigated. The fan-shaped groove microchannels with the best overall heat transfer performance and excellent mechanical properties. The stress of the fan-shaped grooved truss microchannel is reduced by 76.41 % compared to the smooth microchannel. Three structural parameters were investigated, including the length of the truss in the spreading direction (L_x), the ratio of truss flow downstream length to upstream length (e) and truss rod diameter (d). The performance of the microchannels is reflected by the integrated heat transfer factor and the field coordination number. The individual structural parameters were analysed in a single-factor comparison. The microchannels exhibited the best hydrothermal performance in the Reynolds number range of 500 ∼ 1300 at L_x = 0.8 mm, e = 3, d = 0.2 mm. When the Reynolds number is 900, the microchannel with the optimal parameter combination exhibits a remarkable enhancement of 234 % in the Nusselt number and an 80 % increase in the integrated heat transfer factor compared to the rectangular microchannel.

This paper numerically investigates the flow and heat transfer characteristics in the primary surface heat exchanger (PSHE) with cross-wavy (CW) structures. The comprehensive performance affected by hydraulic diameters is evaluated. Moreover, the airflow shuttling behavior at the mixing area of CW-type PSHE is discussed, showing rapid heat transfer enhancement. The advection thermal resistance method and local thermal resistance analysis is proposed, while the impacts of longitudinal pitch and flowrates are considered. Results show that the case with a large hydraulic diameter displays much better comprehensive performance at lower flowrates. When raising the hydraulic diameter from 1.58 mm to 15.8 mm, the heat transfer rate per unit pumping power grows by 36.1 %. However, the priority of large channel is gradually disappeared after increasing the flowrates. Meanwhile, the larger longitudinal pitch of the CW channel may result in pronounced improvement on the heat transfer performance due to the presence of airflow shutting behavior at the mixing area as well as the secondary flow near the channel boundary layers. When no airflow shuttling exists, very high advection thermal resistance region can be observed due to the formation of boundary layers. It can be recognized that the case with airflow shuttling behavior can display similar heat transfer improvement compared to that with increasingly high Reynolds numbers, yet the pressure loss is rarely increased.

Gas turbines typically employ combined cycles owing to high exhaust temperature; however, the complex bottoming cycle often constrains system flexibility. In this study, we explored the different methods for utilizing exhaust heat in gas turbine cycles, and found that the residual heat absorption efficiency of the bottoming cycle and power consumption of the compression process were the main factors affecting heat recovery. The closer the approach to isothermal compression, the lower the power consumption of the compression process. Intercooling, a typical approach toward isothermal compression, was primarily constrained by declines in pressure. To address this constraint, we developed a new approach toward coupling the multistage compressed mass storage process, significantly reducing losses in pressure. The resultant decline in pressure during intercooling ranged from 0.01–0.1 MPa, while in this new approach, the decline during heat transfer was < 0.001 MPa. This is a theoretical breakthrough. Meanwhile, coupling the multistage compressed mass storage process increased the thermal efficiency of the cycle by 1.34–4.5 % compared to the one-stage intercooling cycle, and by 2.64–8 % compared to the two-stage intercooling cycle. This study thus provided a foundation for constructing gas turbine cycles using direct recuperation.

The effects of transpiration cooling depends on the distribution of micropores in porous materials. The work simplifies the porous medium to a micro-scale pore plate structure that is densely organized, based on the idea of the capillary bundle model. The effects of hole pattern and hole outlet angle on transpiration cooling are investigated using numerical simulation. It is discovered that the hole outlet angle mostly affects the homogeneity of temperature distribution and has minimal effect on the surface cooling efficiency. The temperature uniformity index dropped by 35.9% yet the surface cooling efficiency only declined by 1.1% when the outlet angle was lowered from 45° to −45°. Furthermore, the temperature uniformity and cooling efficiency are directly affected by the hole pattern. When the long axis of the elliptical hole is parallel to the mainstream, it can achieve the best temperature uniformity; however, when the long axis is perpendicular to the mainstream direction, it can achieve higher cooling efficiency. Third, the material’s permeability will be decreased to varying degrees depending on the hole pattern and hole outlet angle. The results have important reference significance for the design of porous materials used for transpiration cooling.

Dielectric liquid spray cooling is a promising way to dissipate heat of high-power electronic devices. Surface modification is a most cost-effective method to enhance spray cooling. Inspired by leaf veins, this paper designs and fabricates macro-scale, micro- and nano- scale, and multi-scale structured surfaces for dielectric liquid spray cooling. The cooling characteristics are tested on a two-phase spray cooling system using HFE-7100. The results reveal that the heat transfer is enhanced on all the structured surfaces. Two bionic leaf vein structures, reticulated veins and parallel veins, are designed for macro-scale structured surfaces. The results show that the former one is superior to the other thanks to its better liquid distribution. For the micro- and nano- scale structured surfaces, due to the larger surface area and higher thermal conductivity, the graphene coating outperforms the carbon nanotube coating in heat transfer. Multi-scale structured surfaces, featured with leaf veins and micro- and nano- coatings, further enhance heat transfer. The heat flux increases by 116 % compared with that of the smooth surface. The evaporation efficiency reaches 60 % at the surface temperature of 80 °C. Furthermore, the effect of surface temperature on the enhancement ratio of heat transfer is analyzed, revealing various enhancement mechanisms of different scaled structured surfaces.

This work proposes a novel surrogate model (noted as HCP-PIGN) combining two groups of neural networks: i.e., the physics-informed and the graph convolutional neural networks (noted as PINN and GCN). It aims to tackle the existing challenges: pixelated pre-processing of data and large amounts of training data. For predicting 2D steady-state heat conduction, the GCN acting as the prediction module, considering the interdependence between unstructured and neighboring nodes. The PINN serving as the physical constraint module, embeds governing equations into the neural network’s loss function. The HCP-PIGN model obtains precise predictions with diverse geometries and within milliseconds. The predictive performance of HCP-PIGN was further compared with three network structures: i.e., the physics-informed fully connected neural network (noted as FNN), purely data-driven based FNN, and GCN. The results indicate that HCP-PIGN has the lowest error of temperature field predictions, which are below 3 % and 1.3 % for the max and mean relative errors, respectively. The improvements of 28.1% and 34.6% in accuracy are achieved over the pure data-driven GCN, and the physics-driven FNN, respectively. Therefore, the proposed HCP-PIGN model improves the physical prior knowledge and model’s adaptabilities to geometry variations, resulting in superior performances.

In the scientific domain of cooling techniques research utilizing pin-fins, a number of studies have concentrated on the configurations of pin-fins. However, recent investigations have shifted their focus towards the optimization of endwalls. The objective of this optimization is to better control and maintain vortices, which in turn leads to an increase in heat transfer near the endwall. Further research has taken this a step further by optimizing the lower and upper walls of the unadorned heated channel, resulting in a significant boost in heat transfer efficiency. These studies have also led to the discovery of new heat transfer properties and alterations in the flow structure. This research unveils the findings from an examination into the flow field and heat transfer properties of pin–fin arrays featuring a ribbed endwall, specifically referred to as a Discontinuous Ribbed Endwall (DRE). The investigations are executed using Reynolds-Averaged Navier-Stokes (RANS) equations with the k-ω turbulence model at the mesh parameter of the 20.4 million mesh model is used throughout the work. The study involves a numerical investigation of the heat transfer and pressure drop characteristics of the channel, comparing them with the case of flat endwall across a range of inlet Reynolds numbers, spanning from 7400 to 36000. The entire section of the heated channel is divided into 7 upper surfaces, 7 lower surfaces, and cylindrical surfaces to comprehensively investigate the heat transfer characteristics of both pin-fins and endwalls. The results reveal that the heat transfer regions at the pin-fins and endwalls are expanded and significantly enhanced, particularly causing notable alterations in the flow structure and velocity field. However, the coefficient of friction also increases. The Area-averaged Nusselt Number ($\overline{\mathrm{Nu}}$) and the Heat Transfer Efficiency Index (HTEI) improves from 42.99% to 88.65% and from 36.81% to 73.66% for the DRE compared to the case of flat endwall across the entire range of Reynolds numbers. With Reynolds number 21500, when varying the height parameter of the DRE, the maximum value of the HTEI improves by 84.13%. Other geometric parameters of the DRE, including forward width, behind width, left width, streamwise position, and left position, also undergo changes, with the maximum values of HTEI improving by 73.76%, 75.35%, 80.60%, 75.41% and 74.16%, respectively.

We derive explicit formulas for the mean profiles of temperature (modeled as a passive scalar) in forced turbulent convection, as a function of the Reynolds and Prandtl numbers. The derivation leverages on the observed universality of the inner-layer thermal eddy diffusivity with respect to Reynolds and Prandtl number variations and across different flows, and on universality of the passive scalar defect in the core flow. Matching of the inner- and outer-layer expression yields a smooth compound mean temperature profile. We find excellent agreement of the analytical profile with data from direct numerical simulations of pipe and channel flows under various thermal forcing conditions, and over a wide range of Reynolds and Prandtl numbers.

The wall temperature of the high-speed aircraft increases quickly under hypersonic/supersonic incoming flow, which will cause a significant change in the flow structure. To study the transition of the flow type in the supersonic cavity controlled by the wall temperature, numerical simulations are conducted. The cavity length-to-depth ratio (L/D) is varied from 10 to 15, and the wall temperature ranges from 300 K to 1300 K. The results indicate that the type of cavity flow with an L/D ratio of 13 transforms from a closed cavity flow to a transitional cavity flow, when the temperature reaches approximately 775 K. And the transitional temperature rises with the elevated total temperature of the incoming flow. Furthermore, the mechanism of the cavity flow change with wall temperature could be the competition between the recirculation zone and the shear layer in the cavity. The rising pressure with higher wall temperature in the recirculation zone weakens the downward development of the cavity shear layer, preventing it from hitting the cavity floor. As a result, the mass exchange of cavity lip surface, pressure distribution, total pressure recovery coefficient, and heat transfer distribution in the supersonic cavity change dramatically. The critical wall temperature also affected by the sidewall effects and the inflow Mach number.