Wall Heat Transfer Characteristics Research Articles

Classification and wall heat transfer of non-ideal CO2 isothermal laminar boundary layers

The heat transfer properties of non-ideal fluids are crucial in engineering applications. In order to investigate the wall heat transfer characteristics, the self-similar method has been employed to solve supercritical carbon dioxide (SCO2) isothermal laminar boundary layers. The temperature profile regimes are classified into 14 different categories based on their monotonicity and position relative to the pseudo-critical temperature, Tpc. For no trans-Tpc, the boundary layer exhibits subcritical liquid or supercritical vapor, being liquid-like or gas-like region. When free-stream temperature and wall temperature are both lower than Tpc at a large Mach number (Ma), the temperature profile intersects the Tpc twice. In this scenario, the boundary layer is composed of liquid layer, supercritical vapor layer and liquid layer adjacent to the wall, and the wall heat transfer is determined by the thickness of the two bottom layers. At once trans-Tpc, it is seen that the formation of supercritical vapor or liquid film due to pseudo-boiling/condensation plays a key role in the heat transfer deterioration or enhancement. Mainly related to the film thickness and the pseudo-boiling/condensation intensity, the influence of wall temperature and free-stream pressure on the wall heat transfer has been explored by introducing a new dimensionless number Nu*.

Wall reduction and heat transfer characteristics of captured iron ore particles during flash ironmaking process

ABSTRACT The wall reaction and heat transfer characteristics of captured particles in the flash ironmaking reactor are essential to the reduction degree and liquid phase flow. In-situ studies of near-wall reduction were conducted using a high-temperature hot-stage microscope. Furthermore, a mathematical model was established for analyzing heat and mass transfer in iron ore particles. Results showed that above 1642 K, haematite particles partially melted, forming a bilayer structure with a molten ring and unmolten core as the liquid intermediate product spread rapidly. Higher gas flowrates prolonged melting product diffusion time. The liquid intermediate product volume predicted by the mathematical model matched well with the experimental data. Computational analysis revealed fluctuating particle temperatures during the reaction process, with the degree influenced by the reaction rate. Fluctuations increased with higher CO gas flowrate, particle size, and gas temperature, with gas temperature being the most influential factor.

Experimental and computational fluid dynamics modeling of double‐partition divided‐wall column for flow hydrodynamics and wall heat transfer characteristics

AbstractThe aim of this study is to investigate the flow hydrodynamics and wall heat transfer characteristics on the tray of a two‐staggered walled region in a double‐partition divided‐wall column (DPDWC). An experimental setup of DPDWC is designed, and a theoretical temperature gradient (TG) model based on Fourier's equation, which describes the wall heat transfer, is proposed. A coupled computational fluid dynamics‐TG model is developed to analyze the heat transfer characteristics across partitions of DPDWC. The effects of liquid velocity, gas velocity, and different wall thermal boundary conditions on the flow and temperature fields are studied. The results indicate that the lack of a gas cone near partitions led to few vertical backflows of the liquid phase on the tray. Furthermore, the TGs near the liquid phase outlet increased with an increase in the liquid phase velocity. This work provides a scope for design and operation of DPDWC for industrial implementation.

The Effect of Curvature on the Heat Transfer Performance of Regenerative Cooling Passages for a High-Area-Ratio Nozzle

Abstract Numerical studies on the flow and heat transfer characteristics of rectangular regenerative cooling passages with lateral curvature for a high-area-ratio nozzle are presented. Unlike regular geometries of coolant paths used in various engineering applications, high-area-ratio rocket nozzles have steeply curved surfaces over which the cooling passages are provided. Though the inherent inward curvature benefits heat transfer enhancement in the throat region, it is insufficient to circumvent peak heat dissipation demand. Hence, the heat transfer enhancement due to lateral curvature of the coolant fluid passages provided near the throat region is explored in this study. Extensive numerical simulations have been performed to analyze the effect of the geometry of the cooling channel on its flow and heat transfer characteristics. The compressible turbulent flow field inside the nozzle has been resolved to understand the realistic local wall heat transfer characteristics of the typical high-area-ratio rocket nozzle using Advection Upstream Splitting Method (AUSM) scheme-based finite volume solver. Menter’s Shear Stress Transport k–ω turbulence model is used to model the turbulent flow inside the nozzle. Simulations of the incompressible coolant flow and conjugate heat transfer in regenerative cooling passages have been performed with realistic spatially varying local heat flux profiles, resulting due to compressible gas expansion in the convergent–divergent nozzle. Secondary flow structures are formed due to the lateral curvature of the coolant fluid passages and are found to enhance the heat transfer considerably. Further, the effect of coolant flowrate and channel curvature have been examined to explore its suitability to negotiate the peak heat flux dissipation demand at the throat region of the nozzle.

Experimental Study on Wall Temperature and Heat Transfer Characteristics of Gravity Heat Pipe by Heat Input Power and Working Materials

ABSTRACT With the rapid development of the coal industry, the possibility of spontaneous combustion of coal in the process of mining, storage and transportation has gradually increased. Coal in the pile state is very easy to lead to the occurrence of coal spontaneous combustion disaster because of the good heat storage environment and ventilation conditions. In order to study the impact regulation on the heat pipe heat dissipation effect under different heat source input power conditions, the working performance of the heat pipe is investigated by using methanol working material and no working material as a contrast. The experiments were conducted to determine the suitable working heat source environment for the gravity heat pipe by comparing and analyzing the wall temperature of the gravity heat pipe under different heat source input power as well as the use of the working material. The results show that the copper-methanol gravity heat pipe can best control the temperature of the heat source location to continue to rise, destroy its heat storage environment, and suppress the self-heating of the coal pile. The heat dissipation effect of the copper-methanol gravity heat pipe at the heat source input power of 75 W. It shows that different working materials have corresponding working environment. This experimental study is conducted to be able to destroy the self-heating environment of coal in the process of piling, which has certain guiding significance for the improvement of coal pile spontaneous combustion prevention technology.

Effect of nano-sized aluminum additive on wall heat transfer characteristics of the liquid-fueled scramjet engine

Hydrocarbon fuel loaded with highly energetic nanoparticles possesses superior energy density and has great potential to significantly improve scramjet engine performance. The addition of nanoparticles also leads to the increase of combustion temperature and solid deposition on the inner surface of the engine, which affects the wall heat transfer characteristics. In this paper, the stable combustion of JP-10 slurry fuel containing aluminum nanoparticles was achieved in a scramjet engine for the first time, and conspicuous deposition was observed in the engine inner surface. Convective heat transfer models for both pure fuel case and slurry fuel case were established to quantitatively investigate the wall heat transfer characteristics. Calculation results illustrated that the convective heat transfer coefficients of the sensor surface in slurry fuel case are 28.74% higher than that of in pure fuel case, which was the direct reason for the high heat flux results in slurry fuel cases. Further research indicated that the higher wall friction coefficient is the origin of the higher convective heat transfer coefficient. This study found that the rough surface of the deposition in slurry fuel case changes the wall heat transfer characteristics of the engine, resulting in higher heat flux by enhancing the convective heat transfer process.

Detailed Velocity and Heat Transfer Measurements in an Advanced Gas Turbine Vane Insert Using Magnetic Resonance Velocimetry and Infrared Thermometry

Abstract This work reports the results of paired experiments for a complex internal cooling flow within a gas turbine vane using magnetic resonance velocimetry (MRV) and steady-state infrared (IR) thermometry. A scaled model of the leading edge insert for a gas turbine vane with multi-pass impingement was designed, built using stereolithography fabrication methods, and tested using MRV techniques to collect a three-dimensional, three-component velocity field data set for a fully turbulent test case. Stagnation and recirculation zones were identified and assessed in terms of impact on potential cooling performance. A paired experiment employed an IR camera to measure the temperature profile data of a thin, heated stainless steel impingement surface modeling the inside turbine blade wall cooled by the impingement from the vane cooling insert, providing complementary data sets. The temperature data allow for the calculation of wall heat transfer (HT) characteristics, including the Nusselt number distribution for cooling performance analysis to inform design and validate computational models. Quantitative and qualitative comparisons of the paired results show that the flow velocity and cooling performance are highly coupled. Module-to-module variation in the surface Nusselt number distributions is evident, attributable to the complex interaction between transverse and impinging flows within the apparatus. Finally, a comparison with internal HT correlations is conducted using the data from Florschuetz et al. [1981, “Streamwiseflow and Heat Transfer Distributions for Jet Array Impingement With Crossflow,” ASME 1981 International Gas Turbine Conference and Products Show, American Society of Mechanical Engineers. doi:10.1115/1.3244463]. Measurement uncertainty was assessed and estimated to be approximately ±7% for velocity and ranging from ±3% to ±10% for Nusselt number.

Data assimilation for turbulent mean flow and scalar fields with anisotropic formulation

This work presents an anisotropic formulation for data assimilation (DA) of turbulent flows using the continuous adjoint system. This DA scheme serves as a tool to complement flow measurement data (the observations), which are usually limited in spatial range and measurable quantities, with the help of a predictive model in the framework of the Reynolds-averaged Navier–Stokes approach (the estimator). Herein, the measured data profiles at several locations are used as observations. In the estimator, a forcing term $${\varvec{F}}$$ is added to the momentum equation to compensate for the contribution of the anisotropic eddy viscosity, whilst the isotropic part of the eddy viscosity is determined by conventional turbulence models. $${\varvec{F}}$$ is optimised using the continuous adjoint equations, driving the predicted flow quantities towards the observations. Similar treatments are conducted for the scalar prediction. Three test cases are used for assessment and validation of the present DA scheme. The results of a circular jet demonstrate that the mean flow and scalar fields can be perfectly reproduced from the observations due to the $${\varvec{F}}$$ optimisation. The flow over a blunt plate demonstrates the ability of the present DA scheme to reconstruct global flow fields from several measured velocity profiles. The flow and heat transfer on a ribbed wall reveal that once the global flow field is well recovered, the wall heat transfer characteristics can be correctly determined with a limited number of observations. The present anisotropic DA approach is quite practical for complex flow applications, serving as a complement to our previous isotropic adjoint-based data assimilation for further DA work.

Numerical Investigation of Influence of Entropy Wave on the Acoustic and Wall Heat Transfer Characteristics of a High-Pressure Turbine Guide Vane

As an indirect noise source generated in the combustion chamber, entropy waves are widely prevalent in modern gas turbines and aero-engines. In the present work, the influence of entropy waves on the downstream flow field of a turbine guide vane is investigated. The work is mainly based on a well-known experimental configuration called LS89. Two different turbulence models are used in the simulations which are the standard k-ω model and the scale-adaptive simulation (SAS) model. In order to handle the potential transition issue, Menter’s ð-Reθ transition model is coupled with both models. The baseline cases are first simulated with the two different turbulence models without any incoming perturbation. Then one forced case with an entropy wave train set at the turbine inlet at a given frequency and amplitude is simulated. Results show that the downstream maximum Mach number is rising from 0.98 to 1.16, because the entropy waves increase the local temperature of the flow field; also, the torque of the vane varies as the entropy waves go through, the magnitude of the oscillation is 7% of the unforced case. For the wall (both suction and pressure side of the vane) heat transfer, the entropy waves make the maximum heat transfer coefficient nearly twice as the large at the leading edge, while the minimum heat transfer coefficient stays at a low level. As for the averaged normalized heat transfer coefficient, a maximum difference of 30% appears between the baseline case and the forced case. Besides, during the transmission process of entropy waves, the local pressure fluctuates with the wake vortex shedding. The oscillation magnitude of the pressure wave at the throat is found to be enhanced due to the inlet entropy wave by applying the dynamic mode decomposition (DMD) method. Moreover, the transmission coefficient of the entropy waves, and the reflection and transmission coefficients of acoustic waves are calculated.

Wall heat transfer in the inverted annular film boiling regime

Inverted annular film boiling (IAFB) is one of the post critical heat flux (post-CHF) regimes, which may occur during the core reflooding phase following large-break loss of coolant accidents in water-cooled nuclear reactors. This paper first analyzes wall heat transfer characteristics and summarizes convective wall heat transfer coefficient correlations developed for the IAFB regime in the literature. Parametric effects of the inlet subcooling, mass flux, and pressure on the wall Nusselt number in the IAFB regime are studied. It is found based on the experimental data in the literature that the system pressure has negligible effect on the wall Nusselt number while the effects of the mass flux and inlet subcooling are complex and depend on the flow conditions. The modified Bromley model and laminar flow model with an enhancement factor for the IAFB regime, currently used in COBRA-TF and TRACE, respectively, are compared with Stewart’s experimental data under subcooled and low-quality conditions in the IAFB regime. Both of these two models under-predict the heat transfer coefficients calculated from Stewart’s experimental data for high flow and high subcooling conditions. Based on the analysis of the parametric effects, two new wall heat transfer correlations are proposed by correlating the Nusselt number and non-dimensional vapor film thickness using Stewart’s experimental data and benchmarked against the experimental data in the literature. These two new correlations cover ranges of the system pressure from 2.0 to 9.0 MPa, mass flux from 215 to 923 kg/m2-s, and inlet subcooling up to 50 °C.

Gas pulsation effect on flow behaviors and heat transfer in a tapered fluidized bed

A three-dimensional simulation is performed to study the bed to wall heat transfer characteristics in a tapered fluidized bed with pulsed flow by means of the dense discrete phase model (DDPM). A unique core-annulus flow behavior is revealed in a tapered fluidized bed. The heat transfer behaviors are found to be closely associated with the gas-solid hydrodynamics. The impacts of taper angle and heater probe location are also evaluated. It is found that the bed-to-surface heat transfer coefficient can be certainly improved by reducing the taper angle. When the distance between the probe location and the gas inlet becomes closer, the bed-to-surface heat transfer is enhanced. Meanwhile, the effect of pulsating waveform of rectangular flow on fluidization and heat transfer characteristics is also analyzed. The results reveal that the rectangular pulsed flow with a long-time OFF period will sharply deteriorate the heat transfer behaviors in a tapered fluidized bed.

Influence of shock structure on heat transfer characteristics in supersonic under-expanded impinging jets

In this study, the relationship between the shock structure of impinging jet flow and target wall heat transfer characteristics was investigated. In the case of a high Reynolds number impinging jet that can enter the supersonic flow regime, the stagnation temperature of the impinged surface is affected by the jet structure as well as other factors, such as the nozzle to plate distance and radial distance. When the jet flow velocity becomes supersonic, shock structures form at the downstream of the nozzle exit. Complicated shock structures, such as the Mach shock disk and plate shock, are expected to affect the heat transfer characteristics of the impingement surface. In this study, the cooling performance of a supersonic nitrogen (N2) jet is investigated by measuring the impinged surface temperature, and the flow of the jet is visualized by schlieren imaging. The visualized image and surface temperature are compared to clarify the shock structure-related heat transfer characteristics. Under experimental conditions where the nozzle pressure ratio changes to a moderate and highly under-expanded regime, the surface temperature fluctuates according to the changes of jet structures in each condition. According to a position of Mach crossing point, which is an intersection of oblique shock structures, a behavior of jet fluid is decided to be trapped inside the recirculation zone or to be spread to wall jet region. In case that the jet fluid is trapped, the cooling performance of the supersonic jet is significantly reduced and vice versa. Therefore, the heat transfer characteristics can be explained by roles of the jet flow field structures including the recirculation flows and shock locations.

A numerical investigation on heat transfer dynamics of a periodically forced boiling channel in low amplitudes

In this article, the authors report wall-heat transfer characteristics of a periodically forced vertical boiling channel with the help of numerical simulation based on four-equation drift flux model. The periodic forcing on the boiling channel was initiated by the sinusoidal perturbation of the inlet velocity. The effect of the frequency ω and the amplitude ã of the forcing on the heat transfer enhancement were studied to formulate a mechanism for the enhancement of wall heat flux qw″. qw″ increases with ã in the range of ω less than a critical value ωcrit. The comparison of the numerical solution and the asymptotic solution of a reduced-order model through harmonic balance method (HBM) shows that both the solutions for the amplitude of exit mixture enthalpy decrease with ω consistently and asymptotically, when ω is greater than the characteristic frequency ωc of the channel. The enhancement is also facilitated by chaotic instability (mixing) at higher ã and higher external heat flux.

Numerical study on heat transfer by swirling impinging jets issuing from a screw-thread nozzle

To achieve uniform cooling at high heat transfer rates, a novel swirler with some circumferential screw grooves was developed to generate the swirling impinging jet (SIJ). Using the RNG k-ε turbulence model, the behavior of the flow field for the impinging jet in a confined cavity was analyzed, and the effects of jet spacing, swirl angle, and Reynolds number (Re) on the target wall heat transfer characteristics of the SIJ issuing from the screw-thread nozzle were studied. The numerical results showed that the designed swirler was effective to increase the radial uniformity of heat transfer compared to a multi-channel conventional impinging jet. A good result in terms of radial uniformity of local Nusselt number (Nu) was achieved in the case of θ=60° and a suitable Re in the turbulent status for a small jet spacing of h/dj≤3.0.

Wall Heat Transfer Measurements in High-Pressure Combustion Devices

AbstractTo investigate the wall heat transfer characteristics in a combustion chamber over a broad range of pressure conditions, a method of taking a series of wall temperature measurements in a heat sink combustion chamber, and calculating the wall temperature and heat flux from the temperature data, was introduced. The temperature and heat flux at the inner wall of the combustion chamber can be obtained using this method. The key parameters of the test article are presented, and the technique requirements for thermocouples are described. A two-dimensional (2D) axisymmetric numerical calculation method was developed to obtain the wall temperature and heat flux from the measured temperatures. The method was applied to a firing test of a single-element gaseous hydrogen/gaseous oxygen shear-coaxial injector chamber to demonstrate the details of the method. Furthermore, the heat transfer characteristics of a single-element combustion chamber at chamber pressures of 0.92 MPa to 6.1 MPa were investigated using...

A new family of unsteady boundary layers over a stretching surface

In this paper, a new family of unsteady boundary layers over a stretching flat surface was proposed and studied. This new class of unsteady boundary layers involves the flows over a constant speed stretching surface from a slot, and the slot is moving at a certain speed. Depending on the slot moving parameter, the flow can be treated as a stretching sheet problem or a shrinking sheet problem. Both the momentum and thermal boundary layers were studied. Under special conditions, the solutions reduce to the unsteady Rayleigh problem and the steady Sakiadis stretching sheet problem. Solutions only exist for a certain range of the slot moving parameter, α. Two solutions are found for −53.55° < α < −45°. There are also two solution branches for the thermal boundary layers at any given Prandtl number in this range. Compared with the upper solution branch, the lower solution branch leads to simultaneous reduction in wall drag and heat transfer rate. The results also show that the motion of the slot greatly affects the wall drag and heat transfer characteristics near the wall and the temperature and velocity distributions in the fluids.

DEPRESSURIZATION EFFECTS ON THE THERMAL FIELDS AND HEAT TRANSFER DURING HEMI-SPHERICAL BUBBLE GROWTH ON A HEATED SURFACE

A numerical simulation has been carried out which predicts the growth and wall heat transfer characteristics of a bubble atop a heated flat surface in an otherwise quiescent pool of liquid. In accordance with the experimental conditions and observations of Merte et al [29] the simulations are carried out on a constant heat flux surface in microgravity and the bubble maintains a hemispherical shape with no contribution of a microlayer. The model, computational technique and interface tracking methodology provide very high spatial and temporal resolution. This is true for the micron sized nucleus in metastable equilibrium with its surrounding liquid at the end of the measured waiting time, through the surface tension, transition and heat transfer controlled growth domains where the bubble expands to macrosized. The simulations indicate that the rapid depressurisation of the vapour bubble as it expands occurs in conjunction with a like drop in the vapour temperature. This establishes a substantial temperature gradient and subsequent evaporative cooling effect of the heater surface near the moving triple interface. The influence of bulk liquid advection and transient conduction are discussed in relation to the bubble dynamics.

MNM-P3-2 沸騰熱伝達研究へのMEMSセンサの応用(P3 デバイスを用いた熱・流体計測)

This paper describes a novel approach method to investigate heat transfer mechanism of nucleate boiling by use of the MEMS sensor including micro thermocouples, trigger electrodes and thin film heaters. In the experiments, the local temperature variation beneath the isolated bubble was measured with the MEMS sensor at six wall superheats. In addition, the wall heat transfer was quantitatively evaluated by the transient heat conduction simulation in the heating wall with the measured temperature data. Features of used MEMS sensor, measurement results, wall heat transfer characteristics of the isolated bubble boiling and the initial microlayer thickness are demonstrated.

Direct computation of perturbed impinging hot jets

The unsteady flow and temperature fields of an impinging hot jet at a Reynolds number of 1000 and a nozzle-to-plate distance of 6 jet diameters have been obtained by direct numerical solution of the compressible time-dependent three-dimensional Navier–Stokes equations using highly accurate numerical methods. Effects of an external perturbation on the flow and heat transfer characteristics of the transitional impinging jet have been examined. Oscillatory behaviour induced by the external perturbation has been observed for the impinging jet. The external perturbation leads to the large-scale vortical structures in the primary jet stream, which subsequently lead to the strong oscillatory behaviour of the impinging jet. The vortical structures lead to flow transitional behaviour that enhances mixing of the hot jet with the ambient fluid. It has been observed that the wall boundary layer of the impinging jet is thin. Both the instantaneous and time-averaged wall shear and normal stresses and Nusselt number are examined. Although the external perturbation strongly affects the flow structures in the primary jet stream, it does not have significant effects on the wall stresses and heat transfer characteristics of the impinging jet due to the re-laminarization effect of the wall.

FURTHER DISCUSSION ON THE INCOMPRESSIBLE PRESSURE—DRIVEN FLOW IN A CHANNEL WITH POROUS WALLS

The unsteady velocity profiles for a pressure-driven Poiseuille flow in a channel with porous walls under mass transfer is analyzed and solved exactly. The steady state temperature distribution with viscous dissipation is also derived and solved exactly. The influences of mass transfer on transient velocity and state steady temperature profiles are presented and discussed. The wall heat transfer characteristics are also analyzed