Heat Transfer Deterioration Phenomenon Research Articles

Numerical investigation and optimal design of transpiration cooling plate structure for gradient porosity

In this paper, a novel gradient porosity transpiration cooling plate structure (GP-TCPS) is proposed to address the heat transfer deterioration phenomenon owing to non-uniform local temperature distribution in transpiration cooling plate structure (TCPS). The flow and heat transfer (FAHT) of the GP-TCPS were analyzed qualitatively and quantitatively by computational fluid dynamics (CFD) and response surface method (RSM). The effects of various parameters on the injection pressure (Pc) and average cooling efficiency (ηave) of the GP-TCPS were investigated, ultimately identifying optimal structural parameters. The results show that the GP-TCPS significantly improves the temperature uniformity by 20.58–23.78 % compared to uniform porosity transpiration cooling plate structure (UP-TCPS) at low coolant mass flow rates. Compared with the UP-TCPS, the Pc of the GP-TCPS decreases by 84.5–86.2 %, and the deviation of the ηave is 1.3–2.4 %. The RSM indicates that the most important parameters affecting the Pc and ηave of the GP-TCPS are average particle diameter and the thickness of porous media, respectively. The Pc of the optimized structure is reduced by 57.19–77.39 %, the ηave is increased by 7.72–9.42 %, and the temperature uniformity is increased by 52.33–66.14 %. The results provide valuable insights into the mechanism of TCPS and offer promising solutions to mitigate heat transfer deterioration.

Prediction of supercritical CO2 heat transfer behaviors by combining transfer learning and deep learning based on multi-fidelity data

The flow and heat transfer characteristics of supercritical CO2 are important for heat exchanger design and the safe operation of supercritical CO2 power cycles. However, it is difficult to predict the supercritical heat transfer behaviors due to the non-monotonic temperature distribution in the case of the heat transfer deterioration (HTD) phenomenon. For low-cost, fast and accurate prediction of the supercritical heat transfer behavior, this study proposed a transfer learning model based on multi-fidelity data to achieve fast prediction with acceptable accuracy over a wide range of working conditions. This method fully utilized the low fidelity data (empirical correlations) and the medium fidelity data (numerical results) to generate a large amount of data for pretraining, in which the Latin Hypercube Sampling (LHS) method combined with the HTD correlation was used for sampling. For fine-tuning, high fidelity data from experiments was employed. Compared to the deep learning model trained directly with high fidelity dataset, the transfer learning model demonstrated vastly improved predictive performance on both the test and validation datasets. Additionally, the coefficient of determination R2 was discussed to preventing from “physical overfitting”. Instead of excessively pursuing the high R2 (close to 1), the validity of the prediction should be concerned, especially when using the non-smooth experimental data as the dataset for model training. Moreover, the trained models and the relative files are available at Supplementary materials.

Numerical investigation on heat transfer characteristics of S−CO2 in equal-section inclined tubes

The heat transfer characteristics of are investigated at different inclination angles (15°, 30°, 45°, 60°, and 90°) and pressures (p = 8, 9, and 10 ). It is found that the model can more accurately predict the heat transfer of in horizontal tubes. When is large, the top surface temperature will rise sharply. It is due to the gas-like film weakens the thermal absorption efficiency and impedes the fluid in the core region from taking up thermal in the near-wall area, resulting in severe heat transfer deterioration. The buoyancy criterion shows that the buoyancy and the gas-like film thickness decrease as the inclination angle increases, which effectively improves the heat transfer performance in the top area and weakens the heat transfer deterioration phenomenon. Affected by the critical temperature and the peak of specific heat, the bulk temperature also increases when the pressure increases. In the region of = 45° to 315°, raising the pressure is accompanied by higher circumferential wall temperature, whereas in contrast to the change in

Heat Transfer and Flow Structure Characteristics of Regenerative Cooling in a Rectangular Channel Using Supercritical CO2

At an extremely high Mach number, the regenerative cooling of traditional kerosene cannot meet the requirement of the heat sink caused by aerodynamic heating and internal combustion in a scramjet propulsion system. As a supplement of traditional regenerative cooling, supercritical CO2 is regarded as an effective coolant in severe heating environments due to its excellent properties of heat and mass transportation. In this paper, the heat transfer and flow structure characteristics of regenerative cooling in a rectangular channel using supercritical CO2 are analyzed numerically using a validated model. The effect of heat flux magnitude, nonuniform heat flux, acceleration and buoyancy and flow pattern are considered to reveal the regenerative cooling mechanism of supercritical CO2 in the engine condition of a scramjet. The results indicate that the heat transfer deterioration phenomenon becomes obvious in the cooling channel loaded with relatively high heat flux. Compared with the cooling channels loaded with increased heat flux distribution, the maximum temperature increased for the channel loaded with decreased heat flux distributions. When larger acceleration is applied, a relatively lower wall temperature distribution and higher heat transfer coefficients are obtained. The wall temperature distribution becomes more uniform and the high-temperature region is weakened when the coolants in adjacent channels are arranged as a reversed flow pattern. Overall, the paper provides some references for the utilization of supercritical CO2 in regenerative cooling at an extremely high Mach number in a scramjet.

Investigation of the heat transfer enhancement and deterioration induced by vortex generators in low Prandtl number sodium-potassium alloy liquid

Liquid metals, being characterized by low molecular viscosity and high molecular thermal conductivity, appear to be a stupendous choice for heat transfer fluids in deep-space nuclear reactors. Nevertheless, because of space nuclear reactor weight and volume constraints, the heat transfer capability of the heat exchanger must be substantially improved for a deeper space exploration mission. In this study, the influence of the vortex generator applied to the liquid metal was investigated to further promote the heat transfer capability of the liquid metal. And the thermo-fluid behaviors of the sodium-potassium alloy (NaK-78) are compared among the channel with the Body-Centered Cubic (BCC) lattice array, the rectangular ring rib array, and the Kagome lattice array under identical porosity. The numerical results indicate that the heat transfer deterioration phenomena occurred in the case of rectangular ring rib channel, which is a relatively rare occurrence in conventional fluids with the local Nu number even being lower than that of the smooth channel at the same location. Therefore, the rectangular ring rib should be carefully considered and evaluated when it is applied to heat transfer enhancement in NaK alloy fluid. The Kagome structure and the BCC structure could effectively promote the heat transfer performance of the NaK alloy fluid by means of the induced composite vortex system. In comparison to the smooth channel, the average Nu of the cases with the Kagome array and the BCC array yield 17.26–37.32% and 51.98–78.85% higher, respectively.

Determination of the position and characteristics of supercritical heat transfer deterioration

Supercritical fluids (SFs) are widely used in power generation systems using nuclear, solar, geothermal, and fossil energies. The heat transfer deterioration (HTD) phenomenon during heat transfer of SFs is of great importance to the design and operation of the components in supercritical power cycles. The criteria for determining the onset of HTD has been reported in the literature, but the quantitative model to obtain the position of the wall temperature peak during HTD occurrence, and the correlation to accurately predict the heat transfer characteristics at the wall temperature peak, are both missing. In this work, to determine the position and to predict the characteristics of HTD at the wall temperature peak, we developed two correlations based on experimental results for supercritical CO2 (sCO2) in vertical upward tubes with wide range of experimental parameters. The first correlation to predict the location of the peak is developed using experimental data for sCO2, which can accurately determine the dimensionless bulk fluid temperature corresponding to the wall temperature peak. Moreover, a new heat transfer correlation is developed for the Nusselt number at the wall temperature peak, which has a simple form of NuHTD=0.00796Reb0.7215Prb−0.1568K−0.115, where Reb, Prb, and K are Reynolds number, Prandtl number, and supercritical K number, respectively. This correlation shows smaller prediction error compared with five existing correlations in the literature for not only sCO2, but also various other working fluids. It also outperforms the recently proposed K number correlation in predicting characteristics at the wall temperature peak during HTD conditions. The two correlations developed in this work provides the quantitative models for the HTD phenomenon, and can benefit the engineering application of supercritical power cycles.

Mitigation of Heat Transfer Deterioration of Supercritical CO2 Vertical Tube Upward Flows by Introducing Truncated-Ribs in Helical-Like Distribution

Abstract To effectively alleviate the heat transfer deterioration (HTD) phenomenon of supercritical CO2 flow in a vertical circular tube, this paper proposes multiple truncated ribs from a whole O-ring rib but distributed in helical-like distribution. The fluid hydraulics and thermal performance with a verified standard k–ω mode are numerically explored. The effects of the height, the distance, and the number of truncated ribs on flow characteristics, and heat transfer are observed and analyzed in detail. Results show that the heat transfer coefficient increases significantly with increasing rib height, and as the pitch decreases the fluid recirculation area behind each rib decreases, resulting in stronger mixing of swirling flow, which enhances turbulent kinetic energy in the downstream and weakens the buoyancy force, thus mitigating heat transfer deterioration. This study suggests that introducing multiple truncated ribs distributed along helices into circular vertical tubes can be a beneficial way to alleviate heat transfer deterioration and to enhance heat transfer of supercritical CO2 flow.

A Critical Review on Heat Transfer of Supercritical Fluids

Supercritical fluids have been widely used in a variety of applications, in which heat transfer under supercritical pressure is of great importance. This paper presents a critical review on supercritical heat transfer, including a summary on the past work focusing on in-tube heat transfer and the pseudo-boiling concept, the understanding of the mechanisms of supercritical heat transfer, and suggestions for future work. For supercritical fluids heated in tubes, the typical characteristics, the effects of various parameters, the buoyancy and flow acceleration effects, and the heat transfer deterioration phenomenon are all discussed, and the heat transfer correlations and numerical methodologies are summarized. For supercritical fluids cooled in tubes, the experimental observations and explanations, the proposed heat transfer correlations, and the numerical results are summarized and discussed. More importantly, this review provides a comprehensive review of the supercritical pseudo-boiling concept, including the experimentally observed pseudo-boiling phenomenon, the theoretical studies for supercritical fluids to reveal their heterogenous two-phase features, and the progress in the application of the pseudo-boiling concept on supercritical heat transfer. Finally, suggestions for future research are provided to further advance the understanding and enable accurate prediction of supercritical heat transfer.

Modeling of deteriorated heat transfer in supercritical fluids using a two-phase method

In heat transfer processes of supercritical fluids, heat transfer deterioration (HTD) is normally manifested by unusual rise of wall temperature with one or multiple peaks. Despite being dangerous for the heat exchanger equipment, there is a lack of accurate predictions and reasonable explanations of such HTD phenomena. In this work, a two-phase numerical method is validated and optimized in terms of the two-phase model and interval temperature ΔT. The results showed that the Mixture model outperformed the volume of fluid model, where the modeling results were more consistent with the experimental values when ΔT was set as 1 K. In addition, considering the pseudo-boiling theory and the two-phase numerical method, three cases with different forms of HTD were selected for further investigation. The selection of the saturation temperature Tsat and condensation frequency fe was found to also have a significant impact on calculation results. The best predicted results were obtained at a saturation temperature equal to the termination of pseudo-boiling temperature with a fe of around 200. Overall, the two-phase numerical method was found to yield more accurate predictions compared to the single-phase numerical method in this work. Through the analysis of the average gas volume fraction along the flow direction, a link between this fraction and HTD was developed. Moreover, HTD in the case of supercritical fluids could also be divided into two types of pseudo-film boiling, similar to that under subcritical pressure. The first type of HTD is similar to stable pseudo-film boiling where the gas volume fraction is very high at the deterioration point. By dividing the supercritical fluid into three regimes, a good agreement was reported between the wall temperature and the vapor-like regime. Also, the wall temperature and the vapor-like regime were found to peak at the same position. On the other hand, the second type of HTD is similar to the departure from nuclear boiling, which can occur at the inlet or in the low gas volume fraction zone.

Characterisation of observed heat transfer deterioration modes at supercritical pressure with the aid of a CFD model

The paper analyses experimental data exhibiting heat transfer deterioration phenomena at supercritical pressure resulting in quite different trends of wall temperature. The two modes of deteriorated heat transfer considered here are both due to the laminarisation of flow, owing to lighter fluid appearing at the wall as a consequence of heating. However, in one of the two modes, mainly occurring in the liquid-like region at sufficiently large bulk subcooling with respect to the pseudocritical threshold, after a local deterioration heat transfer is restored almost at normal levels, while in the other, at lower pseudo-subcooling and involving at a larger degree also the gas-like region, a jump of wall temperature to a stable deteriorated condition occurs. In the latter case, the wall temperature finally decreases only when the bulk fluid approaches the pseudocritical temperature, thus causing a smaller density difference between bulk and wall, whose increase was at the root of buoyancy effects leading to deterioration.The two heat transfer deterioration modes are here investigated considering the measured trends of wall temperature in two relevant data sets exhibiting the two behaviours and by the use of a CFD model for predicting detailed data. It is shown that while the first mode of deterioration can be interpreted as an entry-length problem, related to strong buoyancy effects, the second mode is complicated by an excursion to a different manifold of what can be defined as a pseudo-boiling curve, occurring when the imposed heat flux cannot be sustained by normal or enhanced heat transfer. In the latter case, an interesting analogy can be considered with phenomena related to the boiling crisis occurring at subcritical pressures, though the causes of such similar behaviours are obviously quite different. The CFD model, validated by comparison with experimental data of wall temperature, is used as a tool to generalise the observed trends and to draw conclusions.The work is carried out in the frame of studies for assessing and possibly improving heat transfer correlations for supercritical pressure fluids, for the use in the future licensing of supercritical water reactors (SCWRs), in the aim to gain better understanding and to suggest possible modelling strategies. Conclusions are proposed in this regard.

Numerical study on the effects of supercritical CO2-based nanofluid on heat transfer deterioration

The drastic variation of thermoproperties of supercritical CO2 may induce heat transfer deterioration (HTD). To solve the HTD problem, the effects of nanoparticles on supercritical CO2 heat transfer performance was numerically studied. Results show that with the increase of nanoparticle concentration, the local temperature peak is further suppressed. The wall temperature has a maximum decrease of 168.9 K, and the optimal heat transfer enhancement of the supercritical CO2 nanofluid has a maximum increase of 51.4%. Detailed flow field analysis indicates that the decrease of density gradient in the buffer layer significantly suppresses the coupling effect of buoyancy and flow acceleration, which leads to better heat transfer performance. Also, the convective heat transfer benefits from the reduced specific heat and improved thermal conductivity of nanofluid. Eventually, it is the high-density-nanoparticle rather than the high thermal-conductivity-nanoparticle that plays a more effective role in the mitigation of the HTD phenomenon.

Turbulence model assessment and heat transfer phenomena inside a rectangular channel under forced and mixed convection

A very high-temperature gas-cooled reactor (VHTR) is an attractive option for hydrogen production due to its high operating temperature. The reactor cavity cooling system (RCCS), a passive safety system of the VHTR, surrounds the reactor vessel with rectangular riser channels and removes decay heat from it. Under emergency operation conditions such as a decrease in the flow rate of the RCCS, the flow condition inside the RCCS riser can become a mixed convection condition accompanying heat transfer deterioration. To design and operate the VHTR with reliable safety, accurate predictions for various convective heat transfer conditions in RCCS riser channels are required. In this study, by comparing the results of an air flow visualization experiment, the predictability of the turbulence models was assessed for the convective heat transfer and complex flow characteristics near the corner region of the rectangular duct confirmed from the experimental study. In this paper, the heat removal rate through the test section of the visualization experiment was quantified under various convective heat transfer conditions, and local temperature measurement data from the experiment were presented to describe the heat transfer deterioration phenomena inside a rectangular riser in turbulent mixed convection. Linear and nonlinear eddy viscosity models were evaluated by comparing the heat transfer coefficient, local temperature, local flow structure including secondary flow, and turbulence quantities obtained from the experiment. Furthermore, improvements to turbulence models for enhancing the prediction of mixed convection heat transfer in rectangular riser ducts were proposed.

Experimental investigation on flow boiling heat transfer of R134a in a minichannel heatsink with copper foam fins

Metal foam is widely used for heat-transfer enhancement. Flow boiling of R134a in a minichannel heatsink with copper foam fins was studied. The heat sink contained four channels that were 503 μm in width and 990 μm in depth, and the copper foam fins had a porosity of 0.8. The mass flux in the experiment ranged from 595 to 1213 kg/m2 s, and the heat flux was in the range of 22–172 W/cm2. Heat transfer coefficients under different mass flow rates, heat fluxes, and qualities were obtained. The heat transfer deterioration phenomenon at different heat fluxes was analyzed. Compared with a solid-fin minichannel heat sink, the heat transfer was enhanced at low heat fluxes, whereas it was weakened at high heat fluxes. The turning point heat flux was between 87 and 100 W/cm2 and increased as the flow rate increased. Mechanisms of the heat transfer deterioration in the copper foam minichannel heatsink were explained by comparing flow patterns in the two heatsinks at the high heat flux.

Investigating of the Heat Transfer Characteristics of Impinging Flow on a Downward-Facing Surface

Critical heat flux (CHF) is an important heat transfer deterioration phenomenon during boiling heat transfer. It has been extensively studied, especially in the field of nuclear energy. Previous research has found that downward-facing heat transfer is worse than upward-facing and should be paid more attention. In this paper, the boiling heat transfer process under different flow rates and inlet distances is investigated. Seven experimental cases were made including a pool boiling case. The experiment studied the effect of inlet distances under small flow rates, which is not covered by previous research. Analysis of the CHF mechanism included surface temperature curves, boiling curves, bubble behaviors, and heat transfer coefficient. The fluctuation of the surface temperature of forced convention cases was observed due to the bubbles sliding along the heating surface. The phenomenon of vapor film fragmentation could also be found. The results show that even at small flow rates, CHF occurring time is postponed and the CHF value increases. Reducing inlet distance or increasing flow rate can both promote boiling heat transfer, thereby enhancing CHF.

Insights into the dynamics of supercritical water flow using a two-phase approach

A two-phase approach has been proposed to study the supercritical flow with heat transfer deterioration (HTD) phenomena so that an analogy can be derived between subcritical flow boiling and supercritical heat transfer. The volume of fluid multiphase model has been used to analyze the flow, and the simulation result reasonably predicts the wall temperature peaks. Moreover, the velocity and turbulent kinetic energy profiles at different axial locations explain the occurrence of HTD. The parametric study of the thermophysical properties revealed that the density variation is the primary cause of HTD in supercritical flows. Leaning onto this observation, the current study focuses on the forces generated due to the density variation. It suggests that for no HTD, buoyancy and inertia forces have to be of comparable magnitude throughout the flow. Mapping of volume fraction variable reveals a sudden jump in the lighter phase thickness near the wall at the site of HTD, which is also reflected as a maximum in the plot of nondimensional two-phase interface distance from the wall (P). However, this observation is only restricted to HTD caused by buoyancy. This can be used to draw analogy with the phenomenon of film boiling in subcritical fluids. In the end, a theoretical expression has been conceptualized for computing the phase boundary distance from the wall (h), which can serve as a fundamental length scale in supercritical flows as it marks the region of highest property gradient near the wall.

Heat Transfer and Secondary Flow Characteristics in a Horizontally Round Pipe for Cooling a Scramjet Combustor by Supercritical n-Decane

Abstract To figure out the abnormal flow characteristics and thermal performance of supercritical fluids, some detailed information of supercritical pressure n-decane flowing in a horizontally round pipe is studied in terms of secondary flow induced by the huge density change or buoyancy force. According to an evaluation of turbulence models, the shear stress transport k–ω is suitable to execute the case of horizontal flow. It is observed that the temperature distributions between the upper wall region and the lower wall region are asymmetric and the location of the maximum buoyancy force coincided with the position of Tpc (pseudo-critical temperature). The generation of a rotating flow arising from the heated wall determines the occurrence of heat transfer deterioration (HTD). In the boom stage of the HTD phenomenon, a dead zone that is close to the upper wall was formed due to the influence of vortices. In contrast, the maximum buoyancy force is located in the core flow zone and it forces the fluid in the mainstream to participate in the cooling process of the heated wall. In addition, the dead zone in the vicinity of the upper wall is broken. This is the main reason why heat transfer deterioration could be inhibited effectively.

Flow and thermal performance of supercritical n-decane in double-layer channels for regenerative cooling of a scramjet combustor

In order to improve the active regenerative cooling performance of a scramjet using supercritical n-decane, a new type of cooling channel, the double-layer channel, is proposed. Based on reliable numerical simulations, temperature profiles, fluid flow track, vortex structure and buoyancy force distribution of three cases are presented, respectively. It is found that the heat transfer performance of the double-layer channel with the same flow direction is better than the other cases and the heat transfer deterioration (HTD) phenomenon is effectively weakened and the wall temperature is greatly reduced. A cross-flow and a large vortex structure induced by the buoyancy forces are helpful to the occurrence of the HTD phenomenon. An “M” type temperature profile and an “M” type buoyancy force distribution are important characteristics of the HTD phenomenon.

Heat transfer deterioration mechanism for water at supercritical pressure

Abstract Heat transfer deterioration (HTD) phenomena in supercritical water is one of the most thermal hydraulic critical issues for supercritical reactors. The onset of heat transfer deterioration and bouncy effect have been studied in details, however limited work has been done on the mechanism of HTD occurrence. In this work, three dimensional analysis of velocity and temperature boundary layers under heat transfer deterioration conditions is conducted. The effect of thermo physical water properties changes inside the fluid boundary layer and its effect on the heat transfer coefficient is studied. Analogy between HTD and critical heat flux condition in subcritical boiling is observed and reported. Under HTD conditions, clotting or sudden stopping of the flow inside the tube is predicted due to the severe variation in density and other thermo physical properties in the tubes radial direction.

Coupled Monte Carlo-CFD analysis of heat transfer phenomena in a supercritical water reactor fuel assembly

In this paper coupled calculations with the CFD code ANSYS-CFX-19.0 and the Monte Carlo neutronics code MCNP6 were performed to analyze the heat transfer in supercritical water flowing through the typical fuel assembly of the high-performance light water reactor (HPLWR), in order to improve the characterization of the heat transfer phenomena in supercritical water under non-uniform axial heat flux distributions that is characteristic of this type of reactor. To check the capability of the CFX model to predict the thermal-hydraulic behavior of supercritical water, the computational results were compared with two experimental data. The Shitsmańs experiment in the presence of heat transfer deterioration (HTD) using four low-Re turbulence models (SST, k-ω, BSL-k-ω, and ω-Reynolds Stress) and the Wanǵs experiment in absence of HTD, using the low-Re-SST and the scalable-wall-function-SSG turbulence models. In the presence of the HTD phenomenon, results showed the high dependency of the wall temperature with the turbulence model and the turbulent Prandtl number selected. In the absence of HTD, both turbulence models studied adequately predicted the behavior of the wall temperature distribution.For the coupled neutronic/thermal-hydraulic analysis of the typical HPLWR fuel assembly, the low-Re SST turbulence model and the Prt = 1.5 were used. Different axial profiles of heat flux generated in the fuel rods were obtained for the different power values studied. For the analyzed conditions, the presence of HTD in the lower zone of the fuel assembly was observed. In addition, the results showed a strong non-uniformity of the circumferential surface cladding temperature distribution in the sub-channel located at the corner of the fuel assembly; a new curvature radius of the assembly box corner was proposed to obtain a well homogenized circumferential wall temperature distribution.

Numerical Investigation of Heat Transfer to Supercritical Water in Vertical Tube under Semicircular Heating Condition

In-depth understanding and analysis of turbulent convection heat transfer of supercritical water under semicircular heating conditions play a major role in system design and security. The inaccurate numerical results on simulating the buoyancy effect under deterioration heat transfer cases are partly attributed to the invalidity of the turbulent model. An improved turbulence model, which is validated suitable to three-dimensional model, is adopted in the present paper to numerical simulated flow and heat transfer in a vertical tube under semicircular heating condition. Heat transfer deterioration phenomenon occurs under semicircular heating condition, while the degree of deterioration is weakened due to the influence of variable physical properties and buoyancy effect. The velocity profile is distorted into “M-shape” in the heating side and present parabolic distribution in the adiabatic side, leading to different deterioration mechanisms under semicircular heating condition compared with uniform heating. The larger density difference between the heating side and the adiabatic side increases the shear stress production of turbulent kinetic energy; turbulent development is much faster recovery than the phenomenon in uniform heating condition. The results show that the semicircular heating condition can effectively alleviate the degree of heat transfer deterioration in a vertical tube.