Heat Transfer Deterioration Research Articles

Characterization and comparative appraisal of two novel annular channel designs with variable flow areas for supercritical heat transfer

Deterioration of heat transfer is a common concern for any supercritical heat transport system, regardless of the geometric orientation. Present study proposes two novel designs of annular channel with variable cross-sectional area and numerically assesses their respective performances, with the primary objective being the enhancement of overall heat transport characteristics in comparison with an equivalent traditional plain annular channel. Both the configurations exhibit substantial gain in terms of overall heat transfer coefficient and prevailing temperature level, while also eliminating deterioration over the entire parametric ranges explored here, with the diverging one demonstrating relatively superior characteristics. The converging channel generally maintains a flatter temperature profile and comparatively lower maximum temperature, and hence can be employed at larger power-to-mass-flux ratios. Taper angle is earmarked as the most influencing design variable, illustrating enhanced performance with greater tapering, albeit at the cost of nominal increase in pressure drop. Both the designs are found to be insensitive to flow acceleration, which is a primary reason of not encountering deterioration. Strong buoyancy effect can be present within the entrance region of the converging design, affecting its overall performance. Local thermalhydraulics have been noted to be contingent to the effective level of turbulence and distribution of specific heat in the radial direction, which also contribute to the favorable response from the diverging design.

Study on combined thermal protection scheme integrating supercritical CO2 regenerative cooling and fuel film cooling used for scramjet engines

Supercritical CO2 closed-Brayton-cycle demonstrates considerable application potential on hypersonic vehicles, effectively addressing power supply issues during prolonged flights. However, the CO2 flow rate is strictly limited by the cycle’s cooling source (aviation kerosene), rendering traditional regenerative cooling inadequate for sufficiently cooling the scramjet engine. To address this challenge, this study proposes an integrated cooling scheme that combines regenerative cooling, ceramic thermal-protective layer, and fuel film cooling. A two-dimensional coupled numerical model was developed, simultaneously considering the convective heat transfer of supercritical CO2 and the shear mixing of supersonic fuel film. The results indicate that the high-enthalpy gas imposes an extreme aerodynamic heat load on the regenerative cooling channel wall, resulting in a notable deterioration in heat transfer for supercritical CO2. Implementing kerosene film cooling within the combustor can effectively mitigate the abnormal heat transfer behavior of supercritical CO2 and significantly reduce viscous dissipation heat within the gas boundary layer. By optimizing the fuel film injection layout, this combined cooling scheme reduced wall heat flux by 43% and temperature by over 400 K compared to traditional regenerative cooling, thus providing effective thermal protection for the scramjet engine.

Supercritical n-decane heat transfer in a vertical tube subjected to high-temperature crossflow air

The study presents a numerical investigation of high-temperature supercritical n-decane flowing upward in a tube subjected to non-uniform heating. The aim of the study is to evaluate the heat transfer mechanisms due to the non-uniform heating condition where it can be observed that the radial inner wall temperatures exhibit a typical pendulum-type distribution. In particular, between 140° < θ < 220° (near the cold side of the tube), the temperature at the inner wall is at the lowest and remains essentially constant, which indicates that the n-decane between the hot and cold sides in the tube exhibits poor mixing. Also, the higher the inlet temperature, the more likely heat transfer deterioration (HTD) will occur closer to the tube inlet. Moreover, detailed information on wall temperatures, velocity, and secondary flow of the supercritical n-decane are evaluated and discussed. The results show that Re is affected by the density and kinetic viscosity of n-decane near the pseudocritical temperature so that Re near the wall fluctuates and has an influence on the wall heat transfer in the HTD region. The Pr at the center of the fluid decreases along the axial direction and the fluid boundary layer becomes thinner, resulting in a more pronounced non-uniformity of Pr in the radial direction. Meanwhile, the stability of the turbulent kinetic energy (TKE) distribution and the high intensity of TKE favor the heat transfer of n-decane in the tube.

Enhancing thermal performance of jet-regeneration composite cooling systems: An analysis of flow mode and distribution utilizing supercritical n-decane and ambient air

To enhance the heat transfer performance of the scramjet, this paper conducts research and analysis on the impact of flow mode and flow distribution of supercritical n-decane and ambient air on flow and heat transfer characteristics, based on regeneration cooling channels. Given the disparities in fluid flow characteristics within the channel, the three flow configurations exhibit varying degrees of heat transfer deterioration. In the jet single outlet flow mode, the fluid mobility within the channel is relatively poor, leading to the most pronounced deterioration of heat transfer. The combined heat transfer performance between the jet fluid and the crossflow fluid is predominantly influenced by the number of jets and the distribution ratio of flow rates. Notably, the jet-crossflow single outlet arrangement exhibits exceptional heat transfer capabilities when the jet flow rate constitutes a relatively low proportion (12.5 %) while the crossflow flow rate is substantial (87.5 %). Ambient air, with its lower density, arrives at the heated surface with significantly higher velocities and greater turbulence intensity compared to supercritical n-decane. As the number of jet holes increases, the inhomogeneity (R) in the Nusselt number gradually diminishes. For most configurations, R is more pronounced in ambient air than in supercritical n-decane.

Study on the heat transfer deterioration of supercritical nitrogen in a vertical tube using a pseudo-boiling model

Heat transfer performance of supercritical cryogenic fluid plays a pivotal role in hypersonic vehicle engine cooling and cryogenic fuel vaporization. Heat transfer becomes highly complicated due to the complex physical properties of supercritical fluids (SFs). Heat transfer deterioration (HTD) occurs in SFs similar to the departure from nucleate boiling of subcritical fluids, resulting in a sudden temperature rise. In this study, a pseudo-boiling model is used to investigate HTD of supercritical nitrogen (SCN2) in vertical tubes. The distribution of the pseudo-vapor phase is analyzed to understand the heat transfer behavior in both normal heat transfer (NHT) and HTD regions. Simulation results show that the accumulated pseudo-vapor film weakens the heat transfer from the wall to the mainstream region, resulting in HTD. The critical heat flux (CHF) of SCN2 is observed in the simulation and verified by experimental data. Further investigation into buoyancy and thermal acceleration elucidates the absence of CHF under large mass flux conditions. The forced convection of high turbulent kinetic energy hinders the accumulation of pseudo-vapor film, resulting in NHT under large mass flux conditions. This paper provides a new approach and insights to investigate HTD of SFs using the pseudo-boiling model.

Experimental investigation on the influence of lubricant oil on CO2 nucleate pool boiling heat transfer characteristics

As a natural working fluid, CO2 is considered the most promising alternative refrigerant to hydrofluorocarbons. In the fields of automotive air conditioning and commercial heat pumps, transcritical CO2 cycles show significant potential for performance enhancement. Although there are many studies on CO2 flow boiling in the open literature, few studies involve CO2 nucleate boiling heat transfer, which is the dominant mechanism of CO2 flow boiling heat transfer process. This study is proposed to conduct experimental investigations of CO2 nucleate boiling heat transfer. The influences of evaporation temperature, heat flux and lubricant oil addition on boiling heat transfer performance and bubble dynamic characteristics are discussed. The results show that in pure CO2 nucleate boiling, heat flux increase leads to higher bubble density and bubble diameter in bulk liquid, which in turn enhances boiling heat transfer. The effect of evaporation temperature increases on bubble diameter is significant. The reduction in bubble diameter weakens the convective heat transfer caused by bubble motion, which leads to less variation in the CO2 nucleate boiling heat transfer coefficient with evaporation temperature. Lubricant oil addition significantly changes the bubble dynamics of CO2 nucleate boiling process, leading to larger bubble density and smaller bubble diameter. Moreover, the oil diffusion at the phase interface notably affects the heat transfer performance, resulting in greater differences in the boiling heat transfer characteristics of the mixtures compared to that of pure CO2. The mixture boiling heat transfer coefficient is collectively influenced by evaporation temperature, heat flux and oil concentration. The experimental results suggest that the heat transfer coefficient of the mixture with an oil concentration of 0.5 % increases by an average of 25 % compared to pure CO2 at an evaporation temperature of 0 °C. At higher evaporation temperatures and high oil concentrations (>1%), oil addition leads to heat transfer deterioration. Findings from this work can provide a better understanding of oil effect on refrigerant boiling heat transfer and a fundamental basis for heat exchanger design of CO2 systems.

Experimental investigation on identification of heat transfer deterioration and optimization of correlations for supercritical CO2

The existing experimental research on heat transfer deterioration of supercritical CO2 (S-CO2) in horizontal heated tubes and how to improve accuracy of heat transfer correlations by identifying this deterioration is still lacking. This paper analyzes the effects of inlet pressure, heat flux and mass flux on heat transfer across various states, with focus on precise heat flux intervals near state transitions. The results indicate that heat transfer at top wall and bottom wall can exhibit significant differences, necessitating independent analysis for accurate understanding and prediction. As heat flux increases during transition from liquid region to liquid-like region, heat transfer coefficient exhibits an upward trend. Both buoyancy and flow acceleration effects have significant impacts on local heat transfer near the pseudocritical region. An optimization method for heat transfer correlations has been proposed and validated. When an optimal value of 0.8 for heat transfer coefficient ratio is set by referencing Dittus-Boelter equation, newly introduced correlation predicts top wall heat transfer coefficient with 95.76 % of data points falling within a ±15 % error margin. The study identifies key factors that influence the deterioration of heat transfer, proposing correlations for identifying critical heat flux and practical measures to mitigate deterioration in industrial applications.

Numerical study of dimpled structures on heat transfer deterioration mitigation with supercritical CO2 heated in vertical tube

To suppress heat transfer deterioration (HTD) in supercritical CO2 Rankine cycle, the flow and heat transfer characteristics of supercritical CO2 in elliptical dimple tubes (ET) are numerically investigated by the Shear-Stress Transport k-ω model in Fluent software. The ET based on a smooth tube with length of 2000 mm and diameter of 9 mm. The operating conditions consist of pressure 8 MPa, mass flux 100–700 kg/(m2∙s), and heat flux 70–300 kW/m2. The results indicate that dimples can prevent sharp decrease in radial density near-wall where HTD originally occurs, which mitigates buoyancy effect and improves heat transfer performance. Compared to staggered dimple, inline elliptical dimple tubes exhibit a better heat transfer. Reducing space (p = 10–30 mm) and increasing rows (n = 3–6) of dimples can further improve the effectiveness. At mass flux 300 kg/(m2∙s) and heat flux 70 kW/m2, the performance evaluation criterion (PEC) increases by 42.7 % and 34.7 % respectively, while the maximum buoyancy value (Bumax) decreases by 52.2 % and 28.7 % respectively. Additionally, the Bumax is highly influenced by ac, which represents the product of dimple width and depth. The optimal ET has a PEC of 2.45 and Bumax of 6.42 × 10-5, begin to be affected by buoyancy effect at Bu ≥ 2 × 10-5 and recovers heat transfer at Bu ≥ 2 × 10-4. A new heat transfer correlation is developed and over 95 % of data falls in a 30 % accuracy.

Temperature‐dependent heat transfer deterioration in ordered graphene/ceramic composites

AbstractBoosting heat transfer of ultrahigh temperature ceramics (UHTCs) with dimensional‐crossover graphene has become an increasing challenge for improving the thermal protective performance of new‐generation spacecraft. Yet its characteristics of anisotropic and high interfacial thermal resistance inevitably cause heat transfer deterioration at high temperatures. Here we develop a few ZrB2/graphene/ZrB2 interface‐dominated diffuse models that are driven by bonding configurations and thermal converger. It has been found that forming stronger bonding configurations by Zr‐C interfacial bonding and designing a preferred thermal converger by ordered graphene are both unarguable ways of suppressing high‐temperature heat transfer deterioration. Each Zr‐C interfacial bonding serves as an independent sluice gate to accelerate heat flow. More high‐frequency phonons are excited at high temperature, which augments the interfacial thermal conductance from 38.5 to 335.4 MW·m−2·K−1 at 700 K. Impressively, a concept of thermal converger inspires us to adopt ordered graphene for changing the diverging behavior in isotropic ZrB2 composites. Its convergent process combines with anisotropic characteristics to transform the isotropic heat flow into the highly ordered graphene. Their thermal conductance can be regulated from minimum (2.5) to maximum (80.8 W·m−1·K−1) by rotating the ordered graphene from 90° to 7.5°. This work paves an optimal way for manipulating the heat transfer of UHTCs.

Experimental study of pool boiling characteristics for non-azeotropic mixtures n-pentane / n-hexane on microstructure surfaces

Mixed alkanes are widely used as refrigerants in applications such as natural gas liquefaction, especially in the Floating Liquefied Natural Gas (FLNG) process. To elucidate the mechanisms underlying the weakening of phase transition in mixed refrigerants and to develop enhanced heat transfer methods, a pool boiling experimental setup is established in this study to investigate the boiling heat transfer characteristics of n-pentane/n-hexane mixtures on three different microstructure surfaces. By combining visual observation of bubble dynamics with analysis of the effects of composition conditions, micro-surface geometry, and wall heat flux conditions on the boiling heat transfer characteristics, it is revealed that the heat transfer coefficient (HTC) on the pyramid surface and the cylindrical surface, compared to the smooth surface, maximum increase by up to 345 % and 203 %, respectively. Compared with pure components, the mixture with a mass fraction of 0.5 n-pentane shows the greatest heat transfer deterioration. Microstructure surfaces significantly enhanced the HTC of the mixed refrigerants. Additionally, the microstructures activate more nucleation sites and higher bubble departure frequency. The guiding effect of the pyramid surface effectively decreases the possibility of bubble aggregation under high Eo and Ga numbers, thereby delaying the formation of mushroom bubbles. Therefore, the pyramid surface shows its advantages at different component ratios. The HTC correlations for pure and mixed refrigerants are developed. The predictions for pure and mixed refrigerants HTCs agree with most of the experimental data with a deviation of ±10 % and ±25 %, respectively.

Experimental investigation of the heat transfer characteristics of CO2 at supercritical pressures flowing in heated vertical pipes

Supercritical CO2 shows great potential as a working fluid in power plant cycles due to its moderate critical pressure of 7.38 MPa and critical temperature of 30.98 °C, closely matching typical heat sink temperatures. Understanding the heat transfer characteristics of sCO2 is crucial for designing cycle components. This publication presents a systematic analysis of sCO2 heat transfer in two heated vertical pipes with 4 and 8 mm inner diameters, with both upward and downward flow at pressures of approximately 7.75, 8.00, and 9.50 MPa, and flow inlet temperatures between 5 and 40 °C, based on 196 experiments. The study investigates a range of mass fluxes from 400 to 2000 kg/m2s and heat fluxes from 10 to 195 kW/m2, resulting in a heat to mass flux ratio of 6 to 275 J/kg. The findings reveal enhanced, normal, and deteriorated heat transfer within the experimental dataset. Buoyancy effects are identified as the main cause of deteriorated heat transfer in the investigated parameter range, with flow acceleration showing no significant influence on heat transfer. A new proposed dimensional criterion categorises 36 out of 119 experiments with upward flow in the deteriorated heat transfer regime, accompanied by temperature peaks rising to 47 K. Thermal inflow lengths range from 0 to 480 inner pipe diameters, with some experiments not achieving a thermally fully developed flow over the entire pipe length. Based on a comparison with approximately 8950 experimental data points, two Nusselt correlations are found, capable of reproducing the experimental results with a mean absolute deviation of around 30 %. This publication provides valuable data for validating numerical models and developing correlations to predict the heat transfer of supercritical fluids.

Research on Safety of Aero-Engine Oil Pipe Under Heating Conditions Based on Fluid-Solid Thermal Coupling.

This paper examines the safety of aero-engine pipelines under different heating conditions. Based on the fire test standard documents, a model of an aero-engine oil pipe was constructed, and its safety under heating conditions that meet the standard was analyzed using fluid-solid thermal coupling. The pipe material was stainless steel 1Cr18Ni9Ti, and the oil inside the pipeline was China RP-3 kerosene. To simulate the different working conditions or pump failure scenarios, various kerosene inlet flow rates were used for the calculations. The results indicate that the pipe wall exhibits an uneven temperature distribution under standard heating conditions. As the kerosene flow rate decreases, the pipe wall temperature rises, and heat transfer deterioration occurs. The increase in the pipe wall temperature reduces the material's strength, while the uneven temperature distribution generates thermal stress, further increasing the safety risk. When the kerosene flow rate is reduced to a certain level, the equivalent stress in the pipe wall exceeds the material's yield strength, leading to a high risk of rupture.

Optimization of evaporation heat transfer in high-porosity porous columns

Compared to pool boiling or flow boiling, the evaporation within porous media can offer a higher heat transfer advantage. In this work, the numerical simulations were conducted on the application of porous columns at hot spots in heat exchanger and an analysis was performed on the transient evaporation of liquid within the porous medium. The thermal performance of porous columns with different porosities and pore sizes were investigated on the basis of the obtained data. It finds that the porous columns tend to experience vapor blockage during the phase change, which will hinder liquid replenishment and result in a deterioration of heat transfer. Moreover, the vapor blockage phenomenon is more pronounced in porous columns with lower porosity and smaller pore size. Six types of porous columns were compared, and it finds that inlet-optimized samples with a relief in vapor blockage exhibits a better thermal performance compared to the conventional ones. After an optimization of outlet, a rapid discharge of vapor within the porous medium column can be achieved, which reduces the difficulty of liquid replenishment and contributes to performance of evaporation heat transfer.

The minimal seed for transition to convective turbulence in heated pipe flow

It is well known that buoyancy suppresses, and can even laminarise, turbulence in upward heated pipe flow. Heat transfer seriously deteriorates in this case. A new direct numerical simulation model is established to simulate flow-dependent heat transfer in an upward heated pipe. The model shows good agreement with experimental results. Three flow states are simulated for different values of the buoyancy parameter $C$ : shear turbulence, laminarisation and convective turbulence. The latter two regimes correspond to the heat transfer deterioration regime and the heat transfer recovery regime, respectively (Jackson &amp; Li 2002; Bae et al., Phys. Fluids, vol. 17, issue 10, 2005; Zhang et al., Appl. Energy, vol. 269, 2020, 114962). We confirm that convective turbulence is driven by a linear instability (Su &amp; Chung, J. Fluid Mech., vol. 422, 2000, pp. 141–166) and that the deteriorated heat transfer within convective turbulence is related to a lack of rolls near the wall, which leads to weak mixing between the flow near the wall and the centre of the pipe. Having surveyed the fundamental properties of the system, we perform a nonlinear non-modal stability analysis, which seeks the minimal perturbation that triggers a transition from the laminar state. Given the differences between shear and convective turbulence, we aim to determine how the nonlinear optimal (NLOP) changes as the buoyancy parameter $C$ increases. We find that at first, the NLOP becomes thinner and closer to the wall. Most importantly, the critical initial energy $E_0$ required to trigger turbulence keeps increasing, implying that attempts to trigger it artificially may not be an efficient means to improve heat transfer at larger $C$ . At $C=6$ , a new type of NLOP is discovered, capable of triggering convective turbulence from lower energy, but over a longer time. It is active only in the centre of the pipe. We next compare the transition processes, from linear instability and by the nonlinear non-modal excitation. At $C=4$ , linear instability leads to a state that approaches a travelling wave solution or periodic solutions, while the minimal seed triggers shear turbulence before decaying to convective turbulence. Deeper into the parameter space for convective turbulence, at $C=6$ , the new nonlinear optimal triggers convective turbulence directly. Detailed analysis of the periodic solution at $C=4$ reveals three stages: growth of the unstable eigenfunction, the formation of streaks, and the decay of the streaks. The stages of the cycle correspond to changes in the linear instability of the turbulent mean velocity profile. Unlike the self-sustaining process for classical shear flows, where the streak is disrupted via instability, here, decay of the streak is more closely linked to suppression of the linear instability of the mean flow, and hence suppression of the rolls. Flow visualisations at $C$ up to $10$ also show similar processes, suggesting that the convective turbulence in the heat transfer recovery regime is sustained by these three typical processes.

Numerical study on heat transfer of two-phase fluid in helical coil pipe under ocean condition

The thermal hydraulic characteristics and heat transfer of two-phase fluid in helical coil once-through steam generator under ocean conditions like rolling and heaving motions are studied numerically. A CFD model is established to analyze the effects of motion parameters like amplitude, period and rolling radius and the heat flux distribution are discussed. The analysis shows that the overall heat transfer decreases under ocean conditions, but the overall deterioration is limited, indicting the helical coil pipe has the stability for ocean conditions. The additional forces like Coriolis, centrifugal and inertial forces introduced by rolling motion are analyzed theoretically and the result shows it detorts the original two main vortexes responsible for high heat transfer due to its spiral geometry, making heat transfer fluctuates up and down periodically near the steady state value. As rolling height increases, the heat transfer is nonlinear due to the competition between Coriolis force and centrifugal and inertial forces. As rolling amplitude increases, the heat transfer variation increases and the local heat transfer deterioration is severely aggravated, indicating high rolling amplitude should be avoided as possible. Increasing rolling period makes heat transfer behavior more approximate to static case as gravity and centrifugal force due to spiral geometry is dominate. Under heaving motion, the overall heat transfer decreases, and increasing heaving amplitude and decreasing period make the fluctuation more severe.

Record-high heat transfer performance of spray cooling on 3D-printed hierarchical micro/nano-structured surface

Managing high-flux waste heat with controllable device working temperature is becoming challenging and critical for the artificial intelligence, communications, electric vehicles, defense and aerospace sectors. Spray cooling, which combines forced convection with phase-change latent heat of working fluids, is promising for high flux heat dissipation. Most of the previous studies on spray cooling enhancement adopted high spray flow rates to strengthen forced convection for high critical heat flux (CHF), leading to a low heat transfer coefficient (HTC). Micro/nanostructured surfaces can enhance boiling, but bubbles inside the structures tend to form a vapor blanket, which can deteriorate heat transfer. This work demonstrates simultaneous enhancement of CHF and HTC in spray cooling by improving both evaporation and liquid film boiling on three-dimensional (3D) ordered hierarchical micro/nano-structured surface. The hierarchical micro/nano-structured surface is designed to coordinate the transport of spray droplets, capillary liquid films, and boiling bubbles to enhance spray cooling performance. Boiling inversion where superheat decreases with increasing heat flux is observed, leading to an ultra-high HTC due to the simultaneous promotion of bubble nucleation and evaporation. Unprecedented CHF is obtained by overcoming the liquid–vapor counterflow, i.e., synergistically facilitating bubble escape and liquid permeation. A record-breaking heat transfer performance of spray cooling is achieved with a maximum heat flux of 1273 W/cm2 and an HTC of 443.7 kW/(m2 K) over a 1 cm2 heating area.

Experimental investigation of non-uniform heating effect on flow and heat transfer of supercritical carbon dioxide:An application to solar parabolic trough collector

In order to reduce the carbon dioxide emission, the supercritical carbon dioxide (sCO2) Brayton cycle is a good choice to convert solar energy into power using solar parabolic trough collectors (PTCs). Because the earth rotation induced non-uniform flux distribution threatens the safe operation of absorber tubes, we investigate the flow and heat transfer of sCO2 in a horizontal tube under non-uniform heating, with the mass fluxes, average heat fluxes and pressures in the ranges of (397–1244) kg/m2s, (99–413) kW/m2, and (7.5–15.7) MPa, respectively. On the contrary to the classical treatment of supercritical fluid (SCF), SCF is treated in a multiphase framework. It is found that, compared with the sCO2 heat transfer in horizontal tubes under uniform heating, the non-uniform heating not only enhances heat transfer, but also increases friction factors. Besides, the non-uniform heating weakens the stratification effect, but the difference of heat transfer between the top and bottom generatrixes cannot be completed eliminated. It is observed that the larger vapor-like-layer thicknesses on tube walls may switch the sCO2 heat transfer from normal heat transfer (NHT) to heat transfer deterioration (HTD). Three heat transfer mechanisms are commented by using the dimensionless numbers regarding the pseudo-boiling in supercritical pressures.

Self-excited oscillation characteristics and mechanisms of supercritical CO2 flowing in a Helmholtz oscillator for enhancing heat transfer

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 (Lc/d1) enlargement results in an increase in outlet pulsation frequency but a decrease in pulsation amplitude. The critical dimensionless ratio of cavity diameter (Dc/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 Lc/d1 = 3 and Dc/d1 = 10.

Heat transfer characterization of SCO2 in non-uniformly heated rectangular cooling tubes under large mass flow and heat flux conditions

A Thermoelectric Conversion System (TECS) with SCO2 as the circulating mass can effectively mitigate the fuel heat sink and electrical energy shortage problems of hypersonic vehicles. However, there is still a lack of research on the use of SCO2 in the harsh thermal conditions in scramjet. In this paper, the heat transfer mechanism of SCO2 in rectangular cooling tubes are numerically analyzed for the first time based on the actual working environment of scramjet. The effects of uneven heat flux (q) axial and circumferential allocations and circumferential and axial tilting of the tube are innovatively considered. Meanwhile, we analyze the heat transfer mechanism of SCO2 under different operating parameters. The results show that the uneven allocation of axial q can suppress the heat transfer deterioration (HTD), and the overall heat transfer coefficient (HTC) can be increased by 11.68 % when the q allocation is Gaussian. The increase of the tube inclination angle can improve its heat transfer performance, and the local HTC can be increased by up to 5.64 % when θ2 = 180°.Increasing the mass flow rate (G) and inlet temperature (Tin) can increase the overall HTC of the tube. This paper can provide guidance for the design and improvement of TECS.

Interpretable machine learning-based prediction and analysis of supercritical fluid heat transfer characteristics at different boundary conditions

The study of the heat transfer characteristics of supercritical fluids is crucial for the safe and economical operation of supercritical energy systems. This study presents a new method to study the heat transfer characteristics of supercritical fluids, employing interpretable machine learning. Two back propagation neural network models are proposed for the prediction of heat transfer to supercritical fluid for the cases with given heat flux and given wall temperature, respectively. The particle swarm optimization algorithm was employed to search for the optimal hyperparameters, and the resulting accuracy was evaluated in comparison with that of the traditional empirical correlation and a number of machine learning models. Combining the SHAP interpretable algorithm with prediction models, the supercritical heat transfer mechanism is explored from the perspective of global and local prediction behaviors based on explainable models. The results demonstrate that the average error of two established neural network models on the test set is 0.47 % and 0.69 %. Furthermore, for vertical upward flow, the buoyancy and acceleration effects are of greater feature importance in heat transfer deterioration. They are the primary factors contributing to heat transfer deterioration behavior. The research method proposed in this study has certain reference significance for further study of the heat transfer characteristics of supercritical fluids.