Wall Temperature Heat Research Articles

Effect of depth and diameter of cylindrical cavity in a plate-type methanol steam reformer on estimated net power of PEMFC

The plate-type methanol steam reformer (MSR) designed for improving its reforming performance is integrated with proton exchange membrane fuel cells (PEMFCs) regarded as a hopeful miniature power source. This study then uses computational fluid dynamics (CFD) through SIMPLE-C algorithm and Arrhenius form of reaction model to study the reforming performance of a plate-type MSR with the cylindrical cavities installed on the bottom of reformer channels. The influences of depth and diameter of cavities, and heated-wall temperature on reforming performance of reformer and estimated net power output of PEMFC have been examined with the same catalyst and inlet conditions in various flow channels of plate-type MSR. The results display that the installation of cylindrical cavities obviously improves the methanol conversion rate and hydrogen yield of a plate-type MSR. Compared with the traditional plate-type MSR, installing the cylindrical cavities of DC = 1.4 mm and HC = 1.5 mm on the plate-type MSR channels leads to the best enhancement in the methanol conversion rate by 22.65% and in the hydrogen yield by 64.52% at Theater = 250 °C. Moreover, the novel plate-type MSR with the cylindrical cavities of DC = 1.4 mm and HC = 1.5 mm acquires the best augmentation of 22.46% in the estimated net power output of PEMFC at Theater = 250 °C.

Experimental investigations on the effect of geometrical properties of helical wire turbulators on thermal performance of a helically coiled tube

As like to straight tubes, experts try to use heat transfer enhancement techniques to improve the thermal performance of helical tubes. Present study provides an experimental investigation on the effect of geometrical properties of spiral wire turbulator on thermal performance of helical tube. Spiral wire is the only kind of turbulator which can be easily inserted into helical tubes. The quality and quantity of helical wire turbulator inside helical tubes are different with strait tubes, due to the presence of centrifugal forces. Besides variation of geometrical properties of spiral wires (including spring pitch, wire diameter and spring diameter) can significantly influence on thermal behavior of helical tubes. In this study spiral wire with different geometrical properties (four different spring pitch and four different wire diameter) is employed and the thermal behavior of helical heat exchanger is experimentally investigated. The helical tube containing spiral wire was put under constant heat flux supplied by means of an electrical heater. In order to calculate the convective heat transfer coefficient, the inlet, outlet and wall temperature of helical tube was measured. The results revealed that variation of the pitch and wire diameter have significant effect on thermal performance of helical heat exchanger. Increment in spring pitch and wire diameter of spiral tube caused increment in Nu (Nusselt) number up to 73% and 70% respectively. Also it was revealed that increment in wire diameter increases the heat transfer coefficient whereas the effect of increment of spring pitch on heat transfer coefficient is vice versa and the increment of spring pitch decrease the heat transfer coefficient.

Thermal hydraulic performance analysis of a post-CHF heat transfer test facility

Post critical heat flux (post-CHF) heat transfer may occur in loss of coolant accidents (LOCAs) in water-cooled nuclear reactors when makeup water quenches the uncovered fuel rods in the reactor core. The post-CHF regimes are characterized by low heat transfer coefficient and high wall temperature due to the vapor-wall contact. A good understanding of the hydrodynamics and heat transfer process in the post-CHF regimes, especially low-quality film boiling, such as inverted annular film boiling (IAFB) and inverted slug film boiling (ISFB), is imperative for accurately modeling the post-CHF phenomena and predicting the fuel rod cladding temperature and void fraction during accidents. This paper first discusses limitations of existing post-CHF hydrodynamics and heat transfer models/correlations and then identifies needs of post-CHF experimental data under high-flow and high-pressure conditions for model improvement. To narrow down the gap in the available experimental data, a Post-CHF Heat Transfer (PCHT) test facility is designed to perform quasi-steady-state low-quality film boiling experiments under high-pressure (ranging from 0.14 to 3.45 MPa), high-flow (ranging from 150 to 2000 kg/m2-s), and high inlet subcooling (up to 50 °C) conditions. In addition to the experimental study, a numerical model using COMSOL Multiphysics tool is developed to inform the test section design and to investigate wall heat flux and wall temperature distributions in the post-CHF regimes. The COMSOL Multiphysics simulation results show that quasi-steady-state post-CHF phenomena can be achieved in the PCHT test section. Considering thermal non-equilibrium conditions in the post-CHF regimes, a one-dimensional two-fluid model is implemented into a computer code to predict the void fraction using the wall temperature information obtained from the COMSOL simulation results. A correlation for the liquid-side Nusselt number is obtained for the IAFB regime based on previous experimental data and is implemented into the code, which results in an improvement in the agreement between the calculated void fraction and corresponding experimental data. With the new Nusselt number correlation, a parametric study of the effects of the heat flux, pressure, mass flux, and inlet subcooling on the liquid-side Nusselt number is studied. In addition, flow regime transition in the post-CHF regimes is studied by comparing the existing correlation and experimental data. Furthermore, the Weber number and liquid-side Nusselt number are calculated to examine whether there exists any correlation between these two non-dimensional numbers and the flow regime transition. The simulation results provide insights for design improvement of the PCHT test section and assist the development of a test plan for future IAFB and ISFB tests under high-pressure, high-flow, and high-subcooling conditions in the PCHT test facility.

Experimental study on the Onset of Nucleate Boiling in a narrow rectangular channel under transversely non-uniform and uniform heating

Onset of Nucleate Boiling (ONB) in a narrow rectangular channel was experimentally studied under transversely non-uniform and uniform heating conditions. The experiment was performed under various mass fluxes and inlet subcooling conditions. The behavior of the bubbles was recorded using a high-speed camera, and was analyzed using an image processing technique. In the case of uniform heat flux, the wall temperature was uniformly distributed in the transverse direction. In the case of non-uniform heat flux, the wall temperature had the lowest value in the middle of the heated surface, and increased along the transverse direction toward the edges. At the same mass flux and inlet subcooling temperature, the thermal power at the ONB under non-uniform heat flux was lower than that under uniform heat flux. However, the local heat flux and wall temperature at the ONB were similar at both heating conditions. In the subcooled boiling region, it was found that the boiling heat transfer under non-uniform heat flux was lower than that under uniform heat flux. Additionally, the slope of the wall temperature-thermal power curve after the ONB was higher in the non-uniform heat flux case. Using the results elicited from image processing, the evaporation and quenching heat transfers were evaluated to analyze the differences in boiling heat transfers. The boiling heat transfer was less affected by evaporation and quenching in the case of non-uniform heat flux.

A circumferentially non-uniform heat transfer model for subchannel analysis of tight rod bundles

The non-uniformity of circumferential heat transfer is small in conventional PWR core but becomes significant with tight-lattice core design for advanced reactors. Predicting the circumferentially non-uniform heat transfer behavior can be challenging given the considerable heterogeneity of the subchannel geometry and the drastic change of property with supercritical fluids. In this paper, a circumferentially non-uniform heat transfer model for subchannel analysis has been developed to predict the circumferential distributions of heat transfer coefficient, wall temperature and wall heat flux. In the model, the sources of the heat transfer non-uniformity are considered to be the circumferentially non-uniform flow area and the fluid property variation. To account for these two effects, new correlation with a non-uniform factor is developed. A series of tests using CFD method was performed for determining the empirical coefficients of the non-uniform factor. Furthermore, a two-dimensional fuel heat conduction model is also added to the subchannel analysis code. The new model was validated by comparing the prediction results with available experimental data of a 2 × 2 square rod bundle with supercritical water. It is demonstrated that the inclusion of circumferentially non-uniform heat transfer model leads to an improvement in the predictive capabilities for current subchannel analysis method and will improve the prediction accuracy of cladding temperatures in the design and safety analysis of reactor fuel elements.

Investigation of urea-water solution spray impingement on the hot surface of automotive SCR system

Selective catalytic reduction is the most promising technique for NOx reduction from diesel engine exhaust emissions. The greatest challenge to implementing the system is solid deposit formation. Spray wall impingement of UWS was investigated in a constant volume chamber by numerical simulation using STAR CCM+ CFD code. The realizable k-e two-layer approach together with standard wall functions and all y+ treatment was implemented for the modeling of turbulence flow. The two-phase flow was modeled by using Eulerian- Lagrangian approach. The heated wall temperature was maintained from 338 K to 573 K during the UWS spray wall impingement. The development of the spray after impinging on the heated wall was visualized and measured. UWS droplet size distribution, wall film formation and droplet evaporation rates were estimated, which are vital parameters for the system performance but not well researched. User-defined functions (UDF) were developed to input the Leidenfrost temperature limit in the calculation and to estimate the desired parameters. The investigation reveals that wall temperature is the most important parameter that significantly affects spray development after impingement, droplet size distribution, wall film formation and droplet evaporation. Increasing the wall temperature leads to longer spray front projection length, smaller droplet size, faster droplet evaporation and smaller film thickness, which are preconditions for urea crystallization reduction. The numerical model and parameters were validated by comparing with experimental data.

Oscillating convective airflow in a vented cavity with a heated immersed body. Influence of the heating intensity

The influence of the air variable properties on the buoyancy-driven airflows established in vented square cavities with an inner heated body is numerically investigated. Two-dimensional, unsteady, and turbulent simulations are obtained, considering uniform wall temperature heating conditions. The low-Reynolds k − ω turbulence model is employed. The average Nusselt number and the dimensionless mass-flow rate are obtained for a range of the Rayleigh number varying from 104 to 1012. The results obtained for different heating intensities are analyzed and compared. The conditions under which the flow becomes clearly transient, giving rise to an oscillatory solution, are determined. The dimensionless oscillating period of the transient Nusselt number exhibits a logarithmic decay as a function of the Rayleigh number. The structure of the flow into the cavity as a function of time, are shown.

Assessment of a lower head molten pool analysis module using LIVE experiment

The main objective of this work is assessment and improvement of the COUPLE module, the lower head molten pool analysis model included in a reactor analysis code RELAP/SCDAPSIM, by using LIVE test series. This module is used to calculate the heat-up of reactor core material that slumped into the lower head of the reactor vessel. Heat transfer in the radial and axial directions to the wall structures and water surround the debris is considered in the module. In the LIVE test facility, the behavior of reactor pressure vessel in the late in-vessel phase of a postulated severe accident in a nuclear power plant is investigated. Due to the recent interest in in-vessel melt retention through external reactor vessel cooling as a key strategy of the severe accident management, the ability and applicability of the module need to be assessed and improved. The heat flux, wall temperature, crust thickness and molten pool temperature are compared with the experimental data. The code was modified to allow modelling external cooling vessel and an input deck for the LIVE facility was created. The results using other empirical correlations are also compared and, as a result, better agreement of heat flux and wall temperature is obtained.

ISPH modeling of natural convection heat transfer with an analytical kernel renormalization factor

The objective of this study is to extend the attention of the incompressible smoothed particle hydrodynamics method (ISPH) in the heat transfer field. The ISPH method for the natural convection heat transfer under the Boussinesq approximation in various environments: pure-fluid, nanofluid, and non-Darcy porous medium is introduced. We adopted the improved analytical method for calculating the kernel renormalization factor and its gradient based on a quintic kernel function for the wall boundary treatment in the ISPH method. The proposed method requires no dummy particle layer to meet the impermeability condition and makes the heat flux over the wall boundary easy to implement. We performed four different numerical simulations of natural convection in cavities with increasing complexity in modeling and implementation: the natural convection in a square cavity with constant differentially heated wall temperature, natural convection with the heat flux from the bottom wall for a wide range of Rayleigh numbers, natural convection in a non-Darcy porous cavity fully filled with nanofluid in different flow regimes, and natural convection in a partially layered porous cavity. The results showed excellent agreement with results from literatures and the in-house P1–P1 finite element method code.

The heat transfer coefficient determination with the use of the Beck-Trefftz method in flow boiling in a minichannel

In this paper, the solution of the two-dimensional inverse heat transfer problem with the use of the Beck method coupled with the Trefftz method is proposed. This method was applied for solving an inverse heat conduction problem. The aim of the calculation was to determine the boiling heat transfer coefficient on the basis of temperature measurements taken by infrared thermography. The experimental data of flow boiling heat transfer in a single vertical minichannel of 1.7 mm depth, heated asymmetrically, were used in calculations. The heating element for two refrigerants (FC-72 and HFE-7100, 3M) flowing in the minichannel was the plate enhanced on the side contacting with the fluid. The analysis of the results was performed on the basis of experimental series obtained for the same heat flux and two different mass flow velocities. The results were presented as infrared thermographs, heated wall temperature and heat transfer coefficient as a function of the distance from the minichannel inlet. The results was discussed for the subcooled and saturated boiling regions separately.

Numerical prediction of compressible heat flow with complex wall temperature in supersonic rocket nozzles

Wall heat transfer coefficients and static wall pressures are determined over wide ranges of stagnation pressures and stagnation temperatures under large pressure gradients in a cooled convergent-divergent nozzle. The effects of specific heat ratio, turbulent Prandtl number and wall temperature value on the heat transfer and on the position of separation flow are not yet dis- cussed accurately. Computing correct boundary layer under adverse pres- sures gradients is of a particular importance to the accurate modeling of separated flow. This numerical investigation is conducted to assess the accu- racy of the SST-V turbulence model when computing boundary layer separa- tion in supersonic nozzle with heat transfer. It is concluded that the wall heat transfer coefficients and the position of separation point are influenced by the variation of many parameters as heat specific ratio, wall temperature and turbulent Prandtl number.

Experimental investigation into the impact of density wave oscillations on flow boiling system dynamic behavior and stability

In order to better understand and quantify the effect of instabilities in systems utilizing flow boiling heat transfer, the present study explores dynamic results for pressure drop, mass velocity, thermodynamic equilibrium quality, and heated wall temperature to ascertain and analyze the dominant modes in which they oscillate. Flow boiling experiments are conducted for a range of mass velocities with both subcooled and saturated inlet conditions in vertical upflow, vertical downflow, and horizontal flow orientations. High frequency pressure measurements are used to investigate the influence of individual flow loop components (flow boiling module, pump, pre-heater, condenser, etc.) on dynamic behavior of the fluid, with fast Fourier transforms of the same used to provide critical frequency domain information. Conclusions from this analysis are used to isolate instabilities present within the system due to physical interplay between thermodynamic and hydrodynamic effects. Parametric analysis is undertaken to better understand the conditions under which these instabilities form and their impact on system performance. Several prior stability maps are presented, with new stability maps provided to better address contextual trends discovered in the present study.

Confined jet impingement boiling in a chamber with staggered pillars

The jet impingement boiling heat transfer is investigated experimentally and numerically in a cylindrical chamber with the dense circular pillars, which is a simplified model of the shell-side chamber of the phase-change heat exchanger in a thermoacoustic Stirling engine. The saturated or subcooled liquid water is impinged onto the horizontal pillars from the bottom inlet of the boiling chamber, whereas the two-phase flow is exhausted through the outlet on the top of the chamber. Aside from the measurements of the wall heat flux and wall temperature, the boiling induced bubbly flow patterns are optically observed with a high-speed camera through the transparent window. In the simulation, the boiling heat transfer is predicted with Rensselaer Polytechnic Institute (RPI) boiling model, in which the wall heat flux is composed of three parts. The predicted boiling curve is compared with the measured one, and the calculated distributions of the vapor volume fraction adjacent to the boiling wall are compared with the captured images. The reasonable agreements have been reached. Three different regimes are identified based on the captured images. The jet velocity plays less significant role in the low heat flux cases for the saturated jets, but more in the high heat flux scenarios for the subcooled jet impingement boiling. The latent heat of phase change weakens the effect of jet velocity when crossing the boiling point. Moreover, a higher velocity of the saturated jet results in a later transition to the fully established boiling regime with a higher wall superheat. After the transition point, the predicted wall heat flux components are almost independent of wall heat flux or wall superheat, indicating that they are the good indicators to determine the boiling regime in the simulation.

Effect of surface radiation on natural convection in an asymmetrically heated channel-chimney system

In this paper, a more realistic numerical approach that takes into account the effect of surface radiation on the laminar air flow induced by natural convection in a channel-chimney system asymmetrically heated at uniform heat flux is used. The aim is to enrich the results given in Nasri et al. (Int J Therm Sci 90:122–134, 2015) by varying all the geometric parameters of the system and by taking into account the effect of surface radiation on the flows. The numerical results are first validated against experimental and numerical data available in the literature. The computations have allowed the determination of optimal configurations that maximize the mass flow rate and the convective heat transfer and minimize the heated wall temperatures. The analysis of the temperature fields with the streamlines and the pressure fields has helped to explain the effects of surface radiation and of the different thermo-geometrical parameters on the system performances to improve the mass flow rate and the heat transfer with respect to the simple channel. It is shown that the thermal performance of the channel-chimney system in terms of lower heated wall temperatures is little affected by the surface radiation. At the end, simple correlation equations have been proposed for quickly and easily predict the optimal configurations as well as the corresponding enhancement rates of the induced mass flow rate and the convective heat transfer.

Numerical investigation on pulsating heat pipes with nitrogen or hydrogen

With flexible structure and excellent performance, pulsating heat pipes (PHP) are regarded as a great solution to distribute cooling power for cryocoolers. The experiments on PHPs with cryogenic fluids have been carried out, indicating their efficient performances in cryogenics. There are large differences in physical properties between the fluids at room and cryogenic temperature, resulting in their different heat transfer and oscillation characteristics. Up to now, the numerical investigations on cryogenic fluids have rarely been carried out. In this paper, the model of the closed-loop PHP with multiple liquid slugs and vapor plugs is performed with nitrogen and hydrogen as working fluids, respectively. The effects of heating wall temperature on the performance of close-looped PHPs are investigated and compared with that of water PHP.

Time periodic evaporation heat transfer of R-134a in a narrow annular duct due to mass flow rate oscillation

An investigation into the effect of mass flow rate oscillation on the R-134a evaporation heat transportation in a horizontal annular duct was experimentally conducted. The experiments were performed at different amplitudes (10, 20, and 30%) and periods (20, 30, 60, and 120 s) of the mass flux oscillation in a duct with different gap sizes (1.0, 2.0 and 5.0 mm). In this regard, the time variations of evaporation heat transportation coefficient and the heated wall temperature were also analyzed for the thermal characteristics of the oscillatory evaporation heat transfer. Measured results showed that the amplitude and period of the mass flux oscillation insignificantly affected the time-average heat transportation coefficient for the R-134a oscillatory evaporation heat transfer. However, the larger amplitudes and longer periods of the mass flux oscillation led to stronger wall temperature oscillations.

Comparison of RANS and IDDES solutions for turbulent flow and heat transfer past a backward-facing step

Numerical simulations were carried out under conditions of the benchmark-quality experiments of Vogel&Eaton (1985), where nominally 2D fluid dynamics and heat transfer past a backward-facing step in a channel with expansion ratio of 1.25 was investigated at the Reynolds number of 28,000 (based on the step height and the upstream centreline velocity). Comparative computations were performed using an in-house finite-volume code SINF/Flag-S and the ANSYS Fluent, running the codes with same grids. Two approaches were used for turbulence modelling. First, the Menter SST turbulence model was used to perform refined 2D and 3D RANS steady-state computations. The 3D analysis was undertaken to evaluate influence of boundary layers developing on the sidewalls of the experimental channel. The data obtained has resulted in the conclusion that the side wall effects disturbing spanwise uniformity of wall friction and heated wall temperature in the test configuration were of the same order or less than the skin friction coefficient and the Stanton number measurement errors. Then, 3D time-dependent computations were carried out using the vortex-resolving IDDES method being a hybridization of RANS and LES. The IDDES results obtained with the two codes are in a satisfactory agreement, especially for the finer grid of 17.3 million cells. Comparing with the experimental data, the IDDES approach produces the best agreement for the wall friction, whereas the RANS solutions show superiority in predictions of the local Stanton number distribution.

MHD stagnation-point flow and heat transfer past a non-isothermal shrinking/stretching sheet in porous medium with heat sink or source effect

The MHD stagnation-point flow of electrically conducting fluid and heat transfer past a non-isothermal shrinking/stretching sheet in a porous medium in presence of heat sink or source are investigated. The governing equations are transformed by similarity transformation technique and the converted equations are solved numerically by shooting method. Computed results are presented in some figures. It is obtained that the boundary layer solutions of steady flow exist with higher shrinking rate for the presence of magnetic field. Also, similar fact is observed when the porous parameter increases. The similarity solutions for some cases of shrinking sheet are of dual nature; it means that there exists two solutions for higher shrinking rate and the solution is unique for all stretching sheet cases. The impact of variable wall temperature along the sheet is massive on the thermal flow and the temperature field. For direct variation of wall temperature along the sheet heat absorption is noted for first solution in shrinking sheet case and it increases with the introduction of heat source; but, the presence of heat sink controls the heat absorption and it helps to return back to normal heat transfer process. Whereas, smooth zik-zak nature of temperature profiles is found for the inverse variation of wall temperature for shrinking sheet case with heat transfer from the surface to the ambient fluid layer. On the other hand, for stretching sheet case normally heat transfer occurs, but for higher values of heat source parameter and for larger magnitudes of power-law exponent of inverse variation of wall temperature heat absorption is also noticed.

ONB, OSV, and OFI for subcooled flow boiling through a narrow rectangular channel heated on one-side

This paper presents an experimental investigation of the thermal hydraulic thresholds of the subcooled boiling instability in forced convective flow, such as the onset of nucleate boiling (ONB), the onset of significant void (OSV), and the onset of flow instability (OFI). The experiment was constrained to water flows in the upward direction under atmospheric pressure through a narrow rectangular channel heated on one-side having a gap of 2.35mm, a width of 54mm, and length of 566mm. The heated length and width were 300mm and 50 mm, respectively. The experiment was performed over a wide range of inlet temperature (35–65°C), mass fluxes (118–1400kg/(m2s)), and heat fluxes (50–650kW/m2). Two experimental methods were adopted to achieve and identify the ONB, OSV, and OFI: (1) the constant mass flow rate approach and (2) the constant heat flux approach. The results showed consistency between the two methods. The ONB was identified through visualized monitoring using a high-speed camera and by using the slope of the wall temperature deviation method. The ONB was predicted by Jens and Lottes’ correlation. The OSV was detected using the high-speed camera and the wall temperature–heat flux curve. Based on the experimental method, the OFI was identified using pressure drop and inlet pressure fluctuation. The data showed that the OSV and OFI could occur at similar points based on the experimental conditions. Furthermore, the OSV models, such as the modified Bowring model and Saha and Zuber’s model, can be used to predict the OFI. Additionally, some OFI correlations showed good agreement with the present data. However, many other correlations underestimated the OFI results, which might have been a result of differences in the experimental parameters. Therefore, a new empirical correlation that predicted the present data and other experimental data within very good accuracy was suggested. The present study describes and discusses the influence of the experimental parameters; inlet subcooling, mass flow rate, and the imposed heat flux on the ONB, OSV, and OFI incipience.

Assessment of RANS turbulence models for numerical study of laminar-turbulent transition in convection heat transfer

In this study, the prediction of different turbulence models for simulation of transition heat transfer in forced convection over flat plate and natural/mixed convection between two flat plates was investigated. These turbulence models included Spalart-Allmaras (S-A), Low-Re k-ε, Re-Normalisation Group (RNG) k-ε, Low-Re k-ω Shear Stress Transport (k-ω SST), k-kL-ω, γ-Reθ and V-SA transition models. In forced convection flow over horizontal flat plate, variation of skin friction coefficient and Stanton number with local Reynolds number, Rex, were studied. The simulations showed that in forced convection V-SA, γ-Reθ and k-kL-ω transition models can closely predict the critical distance of flow transition from laminar to turbulence regime comparing to other turbulence models, while in natural convection k-kL-ω and k-ε models provided better prediction. In the case of mixed convection, the effects of different parameters such as heated wall temperature, inlet velocity, inclination angle and Richardson number on fluid flow and heat transfer were investigated in details. It was found that the changes in the transition point depends on the relative strength of the natural to forced convective flow and heat transfer. The transition point was delayed when the inclination angle of the channel was increased while increase in Richardson number promoted the transition to turbulence. In addition, Low-Re k-ω SST and RNG k-ε predicted a shorter transition region, while γ-Reθ model also predicted a delayed transition point.