Heat Transfer Deterioration Research Articles

Numerical analysis on heat transfer enhancement of sCO2 in the tube with twisted tape

Supercritical carbon dioxide (sCO2) has the advantages of easy to reach critical conditions, inactive chemical properties and good safety, which can be used as reactor coolant. However, the thermophysical properties of sCO2 change drastically near the pseudo-critical point. The resulting buoyancy and flow acceleration effects may cause local heat transfer deterioration in the coolant channel. Twisted tape is a good structure for heat transfer enhancement in tube. It can enhance turbulence, generate secondary flow, and disturb fluid flow boundary layer and thermal boundary layer, which can effectively mitigate the heat transfer deterioration of sCO2. The numerical analysis of sCO2 heat transfer deterioration and the heat transfer enhancement of twisted tape is presented in the article using ANSYS Fluent. The default turbulent Prandtl number model was modified to improve the calculation of turbulence generation and diffusion. The heat transfer enhancement characteristic of sCO2 with twisted tape under different working conditions was considered, and the effects of geometric characteristics were also discussed. The results indicate that when the flow rate is low and the heat flux is high, adding twisted tape can effectively suppress the heat transfer deterioration by reducing the viscous sublayer thickness and enhancing the turbulence intensity near the wall. The smaller the twist ratio, the more obvious the enhancement of heat transfer.

A shape function approach for predicting deteriorated heat transfer to supercritical pressure fluids on account of a thermal entry length phenomenon

The paper addresses the problem of predicting deteriorated heat transfer to supercritical pressure fluids, trying a novel approach whose main features are obtained by consideration of the experimental trends of a meaningful series of experimental data. This approach takes into consideration the modifications that the axial velocity profile undergoes at the entrance of a bare tube, when a supercritical pressure fluid is heated at the wall and decreases its density owing to thermal expansion, giving rise to buoyancy effects. Combining the observation of measured wall temperature trends with their successful predictions by CFD RANS models, the effects giving rise to flow laminarization are firstly interpreted as a very peculiar form of the entry length problem applicable to supercritical pressure fluids. Then, their quantitative prediction is addressed by the use of an iterative procedure based on shape functions tailored on the observed trends of the Nusselt number and of the consequent wall temperature trends. In this frame, the results of a recently developed fluid-to-fluid similarity theory for heat transfer at supercritical pressures provide indication about the relevant dimensionless numbers to be involved in determining the shape functions. Being based at the moment on a series of water experimental data collected at 25 MPa for vertical upward flow, the methodology may suggest a way for predicting heat transfer deterioration in the proposed supercritical water nuclear reactor conditions, providing an additional means to tackle this critical problem by a different approach that may result useful as a practical means to bound expected wall temperature trends.

Numerical investigation of pyrolysis and surface coking of hydrocarbon fuel in the regenerative cooling channel

Fuel pyrolysis is a common phenomenon in regenerative cooling channels. However, the alkenes and aromatic hydrocarbons produced by thermal cracking tend to deposit on the channel surface, leading to local heat transfer deterioration, corrosion resistance reduction, or blockage. This study establishes a numerical investigation at a supercritical pressure to evaluate the fuel flow effects on thermal cracking and surface coking. The results indicate a threshold of fuel flow rate or conversion rate that can improve the utilization of chemical heat sink. The threshold of the conversion rate for RP-3 fuel (Rocket Propellant No. 3 jet fuel) is around 80%, exceeding which will reduce the benefits of fuel endothermic pyrolysis. Moreover, a critical criterion of buoyancy parameter has been proposed for the severe pyrolysis of RP-3 fuel. In addition, the variation of fuel flow significantly influences the distribution of wall temperature and coking precursors, which further affects the coke layer thickness.

Comparisons of Supercritical Loop Flow and Heat Transfer Behavior Under Uniform and Nonuniform High-Flux Heat Inputs

The heat transfer characteristic of supercritical water is one of the crucial issues in SuperCritical Water-Cooled Reactors (SCWRs). The efficiency and safety of the SCWR system are largely dependent on the local heat transfer performance. This paper establishes the numerical model for supercritical water in a long vertical circular loop (inside diameter = 10 mm) and analyzes the flow and heat transfer mechanism during the transition process from subcritical to supercritical states under various heat fluxes (uniform and nonuniform). The results reveal that the difference in thermophysical properties between the boundary layer and the core region is the main reason for the heat transfer behavior, especially during the transition from subcritical to supercritical and liquidlike to gaslike. The flow structure on the buffer layer is a dominating factor for heat transfer deterioration. The cases under variable nonuniform heat fluxes have a higher heat transfer coefficient compared with uniform heat fluxes. But, this will cause large changes of the parameter locally. The dominating factors of heat transfer deterioration under these conditions are also identified.

Spent fuel simulation during dry storage via enhancement of FRAPCON-4.0: Comparison between PWR and SMR and discharge burnup effect

Spent fuel behavior of dry storage was simulated in a continuous state from steady-state operation by modifying FRAPCON-4.0 to incorporate spent fuel–specific fuel behavior models. Spent fuel behavior of a typical PWR was compared with that of NuScale Power Module (NPM™). Current PWR discharge burnup (60 MWd/kgU) gives a sufficient margin to the hoop stress limit of 90 MPa. Most hydrogen precipitation occurs in the first 50 years of dry storage, thereby no extra phenomenological safety factor is identified for extended dry storage up to 100 years. Regulation for spent fuel management can be significantly alleviated for LWR-based SMRs. Hydride embrittlement safety criterion is irrelevant to NuScale spent fuels; they have sufficiently lower plenum pressure and hydrogen contents compared to those of PWRs. Cladding creep out during dry storage reduces the subchannel area with burnup. The most deformed cladding outer diameter after 100 years of dry storage is found to be 9.64 mm for discharge burnup of 70 MWd/kgU. It may deteriorate heat transfer of dry storage by increasing flow resistance and decreasing the view factor of radiative heat transfer. Self-regulated by decreasing rod internal pressure with opening gap, cladding creep out closely reaches the saturated point after ∼50 years of dry storage.

Modeling of heat transfer in a narrow rectangular channel with partial swelling blockage

In plate-fuel-assembly reactor cores, owing to the cladding swelling caused by irradiation damage under high burnup conditions, narrow rectangular channels may be partially blocked. The partial blockage will alter the flow channel shape and affect the heat transfer characteristics. Thus, it is important to focus on the flow and heat transfer characteristics under partial blockage conditions. Accordingly, in this study, the heat transfer at the entrance region of a narrow rectangular channel with protrusions is numerically investigated. Further, variations in the local Nusselt number and fluid field distribution are studied. Results reveal that the protrusions lead to a wake effect that influences the local heat transfer in the blockage region and wake zones. At the upwind face of the protrusions, the local heat transfer increases to a crest value over the hydraulic diameter. A low-speed recirculation region is also formed over a short distance at the intermediate leeward face of the protrusion, resulting in local deterioration of the heat transfer. Moreover, in the wake zone, heat transfer is enhanced owing to the disturbance of the flow streams with different velocities. The effects of the blockage ratio, number, and interval of protrusions on the local heat transfer are also analyzed. This augmentation of the heat transfer decreases with an increase in the Reynolds number, whereas it increases with the height ratio of the protrusions. Additionally, a multiple regression analysis method is employed to analyze the numerical simulation data. Based on these numerical investigations, dimensionless correlations are proposed to estimate the local Nusselt number for rectangular channels with and without partial swelling blockages. The predicted results are in good agreement with the experimental data and reference correlations. For Eqs. (20) and (22), the major points are located within the relative deviation limit of ± 15%, while for Eq. (21), all the data points fall within the relative deviation limit of ± 20%. Comparison results indicate that the correlations can be used to understand the heat transfer upstream and downstream of partial blockages. Furthermore, numerical investigations and qualitative analyses of the effect of spare protrusions on the local heat transfer in the wake area are performed. These spare protrusions lead to local heat transfer deterioration in the wake zone; notably, the spacing between these protrusions does not have a significant influence on the augmentation of the heat transfer downstream.

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.

Experimental investigations on transient flow and heat transfer characteristics of RP-3 at supercritical pressure

Variable operation conditions due to the wide speed range of hypersonic vehicles are complicated and are often related to the heat load change of the chamber wall. Thermal and flow response characteristics during flight state change are investigated in this research experimentally by utilizing the electrically heating experiment system. It is revealed that the response time of flow and heat transfer due to step perturbation is longer than that of perturbation. The response time decreases as the initial temperature increases when the fuel temperature is lower than the critical temperature. Once the temperature is higher than critical temperature, the response time begins to increase as the initial temperature increases. A 3D transient model is adopted to help to explain this phenomenon. The results indicate that the reason for the characteristics is the weakening of heat diffusion near the wall due to heat transfer deterioration. This conclusion is beneficial to the regulation of the coolant.

Numerical study of spacer-induced heat transfer impairment in mixed and free convection heat transfer of water upward flow

Spacer effects on mixed and free convection heat transfer of water vertical upward flow in a circular channel are studied numerically. The analyses indicate that the effects of spacer on downstream heat transfer significantly depend on the Bo* number at low flow rates. In particular, under the upward section of heat transfer impairment (HTI) regimes and heat transfer enhancement regimes, the spacer-induced HTI phenomenon appears. The HTC evolution in the spacer downstream is damped oscillation and lower than the smooth channel in some regions. By analyzing the downstream developments of axial velocity and eddy viscosity distributions in detail, it is found that the spacer-induced damped oscillations in the recovery of the downstream boundary layer result in the spacer-induced HTI. The effects of spacer radial structure, blockage ratio, and number are also investigated. The impairment phenomenon exists under both the orifice and disk shape spacer conditions. The maximum impairment degree of the spacer-induced HTI increases in the blockage ratio. Extra spacers significantly delay the HTC ratio valley but only suppress it slightly. All three methods can not eliminate the spacer-induced HTI phenomenon.

Experimental study on the effect of wall roughness on heat transfer characteristics of supercritical carbon dioxide in vertical tubes

The heat transfer characteristics of supercritical carbon dioxide(S-CO2) are vital for S-CO2 power cycles. Previous studies primarily focused on the heat transfer of S-CO2 in smooth or ribbed tubes. Rough walls can affect the heat transfer in turbulent flows. Whereas roughness effects on S-CO2 heat transfer have rarely been studied. The heat transfer characteristics of S-CO2 in tubes (d=4.57 mm,16207≤Re≤91735) with various roughness (1.5 μm∼40 μm) are investigated experimentally in this study. The results reveal that rough wall helps alleviate heat transfer deterioration, which is embodied in: the critical heat load(q/G) for heat transfer deterioration increases from 0.11 kJ/kg to 0.17 kJ/kg as roughness increases. Heat transfer deteriorates in all tubes under high q/G, the temperature peak and deterioration length drop from 106.9 °C to 73.7 °C and 0.75 m to 0.25 m as roughness increases. No heat transfer deterioration in various tubes under low q/G, the heat transfer coefficient practically triples with increasing roughness. The effect of operation parameters on heat transfer performance of S-CO2 in various tubes is similar. A heat transfer correlation for rough tubes is proposed based on 5460 experimental data, which catches 90% of experimental data within ±20% error.

Evaporation of ultra-thin water film on hot spot with nanopillar array

For the ultra-thin film evaporation, a hot spot was formed by a local dry out, resulting in the heat transfer deterioration. In this work, the stable evaporation of ultra-thin water film on hot spot with designed nanopillar structure was studied based on the non-equilibrium molecular dynamics (MD) simulation. The variation of molecular motion trajectories, evaporation rate and number of migrating water molecules in different regions were recorded over time in the simulation. The results show that the nanopillar array on the hot spot can attract the surrounding liquid to wet the overheated region due to the strong solid-liquid interaction, both the evaporation stability and heat transfer performance can be improved. By comparing three surfaces with different nanopillar arrays on hot spot, it shows that the evaporation rate is one important factor affecting the amount of liquid replenishment: with the increasing number of nanopillars, both the evaporation performance and liquid replenishment capacity are improved. Moreover, the effect of two designed nanostructure configurations on the evaporation performance was investigated. Compared to the configuration of only nanopillar array on the hot spot, the configuration of nanopillar array on the whole surface has a lower temperature for the nanopillars and delays the occurrence of dry out, while the averaged evaporation rate may be reduced as well. Therefore, there is a trade-off between the surface temperature reduction and evaporation rate enhancement. The findings in this study can provide a guidance for the thermal design for the hot spot in the chip cooling technique under the high heat flux application.

Experimental investigation of near-critical CO2 heat transfer performance in a closed natural circulation loop

The natural circulation system based on supercritical CO2 has attracted attention due to its simple structure and safety compared with the forced circulation system. This work presented an experimental investigation of the heat transfer characteristics of CO2 based on natural circulation. A closed loop natural circulation experimental system was built, which was specially designed for near-critical pressure experiments. The effects of filling ratio, coolant temperature, heat transfer rate and system pressure were carefully discussed. Experimental results illustrated the loop had excellent heat transfer capability at subcritical pressure, but the heating section showed heat transfer deterioration at supercritical pressures. The heat transfer behaviors were analyzed. From subcritical pressure to supercritical pressure in the loop, the heat transfer mechanism transforms from two-phase transition to supercritical single-phase convection. Gas-like layer and insulating layer will form near the wall in the heating section, resulting in a large thermal resistance at the wall for heat to enter the core flow.

Numerical study of convective heat transfer to supercritical CO2 in vertical heated tubes

Supercritical heat transfer is important to design and operate advanced power generation system. Non-uniform heating is frequently encountered such as for sCO2 boiler and solar receiver applications. Here, 3D numerical simulations are performed in turbulent regime, coupling solid wall and sCO2 in tubes. Non-uniform heating is simulated by applying heat via a thin metal layer at the outer tube wall, covering half circumference of tube wall. Calculation results matched experimental data well. Heat transfer is compared between uniform heating and non-uniform heating. Numerical results are re-processed by the pseudo-phase transition theory, including a liquid-like region in tube core and a gas-like layer near the tube wall. The two layers of structure are interfaced by a terminating temperature T+ which is determined by thermodynamics. Hence, supercritical heat transfer is evaluated based on the relative importance of thermal resistances induced by the liquid-like region and the gas-like film region. It is found that wall temperatures for uniform heating are higher than those for non-uniform heating, which is explained by larger thermal resistance of the gas-like film for uniform heating than that for non-uniform heating. This difference ensures better heat transfer for non-uniform heating than uniform heating. We conclude that in turbulent regime, the gas-like film dominates the magnitudes of wall temperature and the occurrence of heat transfer deterioration for both uniform and non-uniform heating. Our work is important for heat transfer facilities in supercritical domain, especially under non-uniform heating condition.

Steam Temperature Characteristics in Boiler Water Wall Tubes Based on Furnace CFD and Hydrodynamic Coupling Model

With the development of power plant units of higher capacity and with improved parameters, the proportion of high-capacity units for generating power has increased; this requires large capacity units to take responsibility for power-grid peak shaving. When the boiler operates at low loads, the working fluid in the boiler water wall tubes is subjected to high heat flux in the furnace, which can cause heat transfer deterioration, tube overheating or even leakage. Therefore, it is particularly important to study the reliability of the boiler hydrodynamic cycle during peak shaving (low load operation). This study takes a 1000 MW ultra-supercritical single-reheat Π type boiler with single furnace and double tangential firing as the research object. A furnace Computational Fluid Dynamics (CFD) and hydrodynamic coupling analysis model is established and verified according to the actual operating conditions on site. The calculation results show that the simulated value is in good agreement with the actual operating value. Therefore, the model established in this study can reflect the real situation of the on-site furnace to a certain extent, and has high reliability. Based on the fitting results, the causes of steam temperature deviation of the boiler water wall are analyzed, and measures to reduce the deviation are proposed to provide a necessary basis for power plant operation and boiler design.

Numerical investigation on deteriorated heat transfer of supercritical water flowing upward in tubes with variable cross-sectional geometries

To mitigate Deteriorated Heat Transfer (DHT) of Supercritical Water (SCW) upward flow, tubes with variable cross-sectional geometries are numerically investigated. Three types of variable cross-sectional geometries: converging channel, diverging channel, and periodic geometries, are modelled. Results of wall temperature in the smooth channel are compared with experimental data, and good agreements are obtained. The wall temperature effects of convergent and divergent channels of various proportions and periodic geometries of various amplitudes are investigated. Thermo-Hydraulic Performance Evaluation Criterion (PEC) is proposed to evaluate the performance of the studied geometries using dimensionless parameters Nu/Nu0, f/f0, and PEC = (Nu/Nu0)/(f/f0)1/3. The results indicated that the convergent channels suppress and delay DHT downstream, while opposite effects are observed for divergent channels. Periodic geometries, which have alternating convergent and divergent sections, suppress DHT with minimal pressure drop. 1st temperature peak is reduced by 200 K with 1 mm of outlet radius reduction for the convergent channel. Average PEC is enhanced by 60% with 0.5 mm A of periodic geometry. It is discovered that the convergent section can suppress DHT, and the mitigation effect is proportional to the size of convergence. It is concluded that, since the DHT caused by buoyancy-induced flow laminarization has flattened the cross-sectional velocity profile, the convergent section reverts the laminarization to the normal velocity profile by the nozzle acceleration effect. Nozzle acceleration also increases Turbulent Kinetic Energy (TKE) in the boundary layer which further enhances turbulent heat transfer.

Numerical Investigation on the Heat Transfer of n-Decane in a Horizontal Channel with Axially Nonuniform Heat Flux under Supercritical Pressure

Regenerative cooling is considered promising in the thermal protection of hypersonic propulsion devices such as SCRamjet. However, the heat transfer deterioration (HTD) of hydrocarbon fuel is a severe threat to the thermal structure safety, especially under axially nonuniform heat flux caused by the thermal load difference in different components. In this work, the heat transfer of trans-critical n-decane in a mini-horizontal channel is numerically investigated. The influences of the axially nonuniform heat flux on the heat transfer is focused on. Two types of HTD are recognized and analyzed. The first type of HTD is induced by the near-wall flow acceleration and the local thickening of the viscous sublayer. The second type of HTD is closely related to the expansion of the low thermal conductivity λ and specific heat cp region, which is seriously worsened under axially nonuniform heat flux, especially when the heat flux peak locates where Tw ≥ Tpc. The minimum HTC deteriorates by 40.80% and the Tw_max increases from 857 K to 1071 K by 27.5%. The maximum fluctuation in pressure drop is 6.8% in the variation in heat flux distribution with Φ = 2. This work is expected to offer a reference to the proper match of fuel temperature distribution and the engine heat flux boundary in SCRamjet cooling system design.

Modeling and analysis of steam-water two-phase flow distribution and wall temperature distribution in parallel heated pipes with different manifold types

In solar tower receiver, boiler water-wall and various heat exchangers, the uneven distribution of two-phase flow often occurs among parallel heated pipes, which seriously reduces the operating efficiency and may further induce heat transfer deterioration and soaring wall temperature. In this paper, a coupling model for the flow distribution and heat transfer process of steam-water two-phase flow in parallel heated pipes was established based on the discrete methods. In the present model, two main improvements were proposed as below. On one hand, the calculated equations for the pressure drop and the phase distribution of two phase fluid in both the combining T-junction and dividing T-junction were established to consider the coupling effects of the inlet manifold and outlet manifold on the flow distribution characteristics. On the other hand, the calculation module for the heat transfer and wall temperature along the branch pipes was also coupled in the present model with considering of different heat transfer regions (saturated nucleate boiling region, liquid deficient region, and superheated steam region) along the pipe. The present model was then verified and used to investigate the effects of system pressure, inlet fluid flow, inlet steam quality and manifold type on the flow distribution characteristics of high temperature and high pressure steam-water in parallel vertical upward pipes, and the effect of the uneven distribution of the gas-liquid two phases on the system safety was analyzed and evaluated. It was found that gas phase preferentially enters the first branch pipe near the inlet of the dividing manifold affected by buoyancy, and this results that the inlet steam quality of the corresponding branch pipe is always obviously higher than that of the subsequent pipes. As the system pressure decreases, the outlet fluid enthalpy distribution and outlet wall temperature distribution among branch pipes become more uneven. Moreover, the outlet fluid enthalpy and outlet wall temperature of the branch pipes closer to the inlet of the dividing manifold increase sharply with the decrease of system pressure. In the U-type parallel pipe system, the branch pipes away from the inlet of the dividing manifold are more prone to wall overheating at the outlet, while the opposite is true for the Z-type parallel pipe system.

ADAPTATION OF TEMPERATURE WALL FUNCTION FOR CALCULATION THE HEAT TRANSFER AT SUPERCRITICAL COOLANT PARAMETERS

To date, the engineering community pays special attention to the study, research and development of methods for predicting the behavior of the coolants at supercritical parameters. This paper discusses the problem of adaptation of the existing universal or specialized tools of thermohydraulic analysis for nonlinear heat transfer problems at supercritical coolant parameters with deterioration of heat transfer. Complexities of prediction the nonlinear heat transfer at supercritical parameters of the coolant by engineering methods of computational fluid dynamics are considered. A simple way to adapt the dual-zone Kader's temperature wall function on the basis of existing probe measurements for carbon dioxide is proposed. The problem of implementation into universal packages of computational fluid dynamics (CFD), which is based on the method of choosing the reference coordinate of the near-wall zone to determine the dynamic speed and dimensionless temperature, is discussed. The example on the ANSYS CFX using shows one of the ways to create a special user procedure, which has an improved tendency to predict the axial temperature profile with deteriorated heat transfer. Calibration and validation of the obtained results on the basis of experimental investigations for vertical pipes and rod assemblies of fuel simulators is carried out in the work. The paper also discusses the features of the proposed implementation, and formed recommendations for the application and further improvement of engineering approaches to predict the heat transfer deterioration at supercritical parameters of the coolant.

Experimental Investigation on Intermittent Operation Characteristics of Dual-Temperature Refrigeration System Using Hydrocarbon Mixture

An experimental rig of a zeotropic mixture separation condensation-based dual-temperature refrigeration cycle is built and the mixture R290/R600a is used as the refrigerant. Compared with a conventional cycle, the proposed refrigeration system demonstrates its application advantages under an on–off operation mode. Furthermore, the on–off periodic operation behaviors of this refrigeration system are experimentally investigated. The influence of a different refrigerant charge, a refrigerant mass fraction, a throttling valve opening, and ambient temperature are explored to evaluate the cyclic operation characteristics. The results reveal that the compressor average power for the duration of the compressor startup increases and the compressor duty cycle first declines then increases with the rise of the refrigerant charge. The average compressor during an on-period decreases from 104.5 W to 79.2 W and meanwhile the compressor duty cycles fluctuates between 72.1% and 96.9% as the R600a-charged concentration increases from 30% to 70%. The average power of the compressor during the on-period and the duty cycle are also sensitive to the freezer valve opening variation. Thus, the minimum energy consumption of 1.60 kWh·24 h−1 is achieved at the refrigerant charge of 300 g, a R600a-charged mass fraction of 50%, and a freezer throttling valve opening of 10%. A higher ambient temperature deteriorates heat transfer during condensation and increases the cabinets’ heat load, the compressor duty cycle, and eventually affects the daily power consumption. Generally, the present study offers an in-depth insight of cyclic operation characteristics of a separation condensation-based hydrocarbon mixture dual-temperature refrigerator under two parallel evaporators’ concurrent cooling process.

Numerical investigation of heat transfer deterioration and the variation of Prandtl number in the main flow area under supercritical conditions

The problem of heat transfer deterioration (HTD) exists in supercritical conditions of the power plant during peak operation. The turbulence model is selected to be applicable to the cross-critical flow field, and the different simulation models results are compared with the Yamagata experimental data. The different inner diameters and heat fluxes have a significant effect on the HTD, accompanied by variation in the Prandtl number change rate. This change can be expressed by the full width at half maximum of the Pr number along the bulk fluid enthalpy. It is analyzed that the HTD is related to the layer difference between the temperature boundary layer and velocity boundary layer, and to the decrease of the thermal conductivity inside the temperature boundary layer near the wall.