Local Bulk Temperature Research Articles

Thermal and hydrodynamic analysis inside a channel partially filled with porous material

AbstractIn this work, the transfer of heat and changes in fluid pressure within a rectangular channel fairly packed with porous material have been studied numerically for various Darcy numbers and dimensionless porous layers. The model was run with the following assumptions: laminar flow, forced convection, isotropic porous material, local thermodynamic equilibrium, constant wall temperature boundary condition, and no thermal dissipation. The study covers a broad range for the dimensionless porous layer, 0 ≤ hr < 1, and the Darcy number, 10−4 < Da < 10−2. The fully developed and developing flow over the channel is investigated in numerical study. It was observed that the inertia effect may be disregarded when Da < 10−4. The local dimensionless bulk temperature distribution, pressure drop, and velocity profiles were all shown to be impacted by the Darcy number and dimensionless porous layers, according to the numerical analysis results. The maximum heat transfer rate was attained when the ratio of the porous layer inside the channel was 0.8, and the pressure gradient was the highest. Partial packing of the channel with a porous material has two advantages: it increases the rate heat transmission rate and results in a much smaller pressure drop than a filled porous medium.

Effect of Strip-Fin Height on Jet Impingement Heat Transfer in a Rectangular Channel at Two Jet-to-Target Surface Spacings

Abstract An experimental investigation of heat transfer performance in a rectangular impingement channel featuring staggered strip-fins was completed. Four configurations were considered to study the effects of varying the strip-fin height (H/d = 1.5 and 2.75) at two jet-to-target surface spacings (z/d = 3 and 6) on the heat transfer, pressure loss, and crossflow magnitude for a long impingement channel with in-line, 4 × 12 impinging jets. Also, the effect of the reference temperature choice, either jet inlet temperature or local bulk temperature, for calculating the local heat transfer coefficients was considered. The regionally averaged heat transfer coefficients were measured at seven Reynolds numbers, based on the jet diameter (10k–70k) utilizing the copper plate experimental method. The empirical correlations were expressed for the area averaged Nusselt number estimation of impingement channels with strip-fin or pin-fin roughness elements. The results showed that the long strip-fin channel with z/d = 3 provided the best thermal performance. The discharge coefficients are similar for all configurations between Rejet = 10k and 50k. The results are compared with the impingement channels with conventional pin-fins. They show that strip-fin channels provide lower pressure drop with marginally better heat transfer coefficients compared to the conventional pin-fin channels. However, when the channel weight is considered, strip-fins would increase the roughness material volume more than the conventional pin-fins.

Effect of a Triangular Configuration of Heat Exchanging Tubes on Local Heat Transfer Coefficient in Bubble Column Reactor

The effect of a triangular configuration of heat exchanging tubes (HETs) on the instantaneous and local heat transfer coefficient (LHTC) has been investigated and quantified in an air-water bubble column (BC) utilising an advanced heat transfer technique. The bundle of heat exchange tubes was designed to cover 25% of the column's cross-sectional area (CSA) to simulate the heat exchanger tubes used in the industrial Fischer Tropsch (FT) process. The local heat flux, surface temperature, bulk temperature, and heat transfer coefficient (HTC) were investigated instantaneously in a Plexiglass BC under a wide range of superficial gas velocity (5-45 cm/s). The obtained results reveal that the presence of a bundle of (HETs) and their arrangement effect the (HTC) in the bubble column, where unsymmetrical profiles for (HTC) were obtained with triangular tube pitch arrangement. Additionally, steeper (HTC) profiles notice were when the triangular tube pitch arrangement was utilized. High (HTC) values were obtained at the core region of the column relative to the wall region under all the studied operating conditions. It was discovered that the local and (HTC) increase as superficial gas velocity increases despite the fact that the bubble column is heavily packed with a bundle of (HETs). The rise in HTCs was higher at the core than at the wall region of the column. For example, at a superficial gas velocity of 45 cm/s and at an axial height of 65 cm (Z/D=5), compared to the wall, the HTC in the core of the column increased by 131 %.

乱流域におけるマイクロチューブを流れるガスの温度回復係数の推定

In this paper, a temperature recovery factor defined as the ratio of the actual temperature rise at the wall to the maximum possible temperature rise were estimated from experimentally measured wall temperatures and pressures to determine the velocity and temperature of gas through an adiabatic microtube. The experiments were carried out using two pairs of stainless steel microtubes with D=528.4 and 754.2 μm. One of a pair was tested for measurements of local pressures to obtain local Mach numbers and bulk temperatures. And the other of a pair was tested for measurements of local wall temperatures. The stagnation pressure with the atmospheric back pressure was chosen in such a way that the outlet flow reached sufficiently choked. Local total and bulk temperatures were obtained with total and static enthalpies determined with the measured stagnation & local pressures, stagnation temperatures, mass flow rates by real gas manner. In order to estimate the temperature recovery factor, the measured wall temperature and the obtained total temperature were normalized by bulk temperature and represented as Mach numbers. The estimated values of the temperature recovery factor were independent of the Mach number. The present estimated temperature recovery factor was compared with numerical and experimental ones obtained with the assumption of an ideal gas.

Advanced two phase flow model for transient molten salt receiver system simulation

In order to realistically predict and optimize the actual performance of a concentrating solar power (CSP) plant sophisticated simulation models and methods are required. This paper presents a detailed dynamic simulation model for a Molten Salt Solar Tower (MST) system, which is capable of simulating transient operation including detailed startup and shutdown procedures including drainage and refill. For appropriate representation of the transient behavior of the receiver as well as replication of local bulk and surface temperatures a discretized receiver model based on a novel homogeneous two-phase (2P) flow modelling approach is implemented in Modelica Dymola®. This allows for reasonable representation of the very different hydraulic and thermal properties of molten salt versus air as well as the transition between both. This dynamic 2P receiver model is embedded in a comprehensive one-dimensional model of a commercial scale MST system and coupled with a transient receiver flux density distribution from raytracing based heliostat field simulation. This enables for detailed process prediction with reasonable computational effort, while providing data such as local salt film and wall temperatures, realistic control behavior as well as net performance of the overall system. Besides a model description, this paper presents some results of a validation as well as the simulation of a complete startup procedure. Finally, a study on numerical simulation performance and grid dependencies is presented and discussed.

ASSESSMENT OF CONJUGATE AND CONVECTIVE HEAT TRANSFER PERFORMANCE OVER A PARTITIONED CHANNEL WITHIN BACKWARD-FACING STEP USING THE LATTICE BOLTZMANN METHOD

This research concerns the quality of heat transfer across a two-dimensional backwards-facing step in the presence of vertical or inclined partitions in a rectangular channel. Convective and conjugate heat transfer input using constant temperature and heat flux were successfully achieved for low Reynolds number values. The actual code based on the double multiple relaxation time of the lattice Boltzmann method was employed to model numerically the physical problems related to the fluid flow and heat transfer, where the obtained results had a good agreement with previous experimental research. During this study, parametric variations were carried out according to the inclination and position of the partitions, as well as the thermal conductivity ratio in the case of the conjugate heat transfer problem. The numerical results show that the involvement of the partitions and the thermal conductivity ratio has a significant effect on the fluid flow and heat transfer. This impact controls the evolution of local Nusselt number, bulk temperature, and skin friction coefficient. The improvement of heat transfer is remarkable by the introduction of vertical or inclined partitions such that the quality of heat exchange reaches about 7%. While the presence of two inclined partitions increases the heat transfer by about 6% in case of constant temperature of studied hot wall. The implementation of constant heat flux for the hot wall increases the quality of heat transfer by about 9%-10% by the presence of inclined or vertical walls, 11%-13% with double inclined partitions.

On the interest of a semi-empirical model for the tooth friction coefficient in gear transmissions

Accurate modelling of friction coefficient is of primary importance in efficiency, vibration and failure analyses of enclosed gear drives. After showing the influence of surface/lubricant interactions on friction, the authors used a semi-empirical model which can take all these aspects into account. Lubricant is modelled as an Eyring–Reynolds fluid and rough surfaces are described with two parameters via a stochastic approach. A specific two-disc machine is used to perform series of friction measurements on smooth and rough discs. Smooth discs allow to operate under full film lubrication and to measure a reference shear stress of lubricant, whereas rough discs reproduce gear tooth roughness and generate a representative value of friction on asperities. The purpose of this present paper is to describe calculations of this physical-based friction coefficient model and to present the experimental process. On the basis of new results, the impact of a surface finishing process is assessed as well as the consequence of calculating friction coefficient based on oil injection instead of local bulk temperature.

Matching Magnetic Heating and Thermal Actuation for Sequential Coupling in Hybrid Composites by Design.

Sequentially coupling two material functions requires matching the output from the first with the input of the second function. Here, magnetic heating controls thermal actuation of a hybrid composite in a challenging system environment causing an elevated level of heat loss. The concept is a hierarchical design consisting of an inner actuator of nanocomposite material, which can be remotely heated by exposure to an alternating magnetic field (AMF) and outer layers of a porous composite system with a closed pore morphology. These porous layers act as heat insulators and as barriers to the surrounding water. By exposure to the AMF, a local bulk temperature of 71 °C enables the magnetic actuation of the device, while the temperature of the surrounding water is kept below 50 °C. Interestingly, the heat loss during magnetic heating leads to an increase of the water phase (small volume) temperature. The temperature increase is able to sequentially trigger an adjacent thermal actuator attached to the actuator composite. In this way it could be demonstrated how the AMF is able to initiate two kinds of independent actuations, which might be interesting for robotics operating in aqueous environments.

Determination of local bulk temperature based heat transfer coefficient in the channel with multiple inlets and multiple outlets

The accuracy of heat transfer coefficient of internal cooling channel is directly related to the selection of the fluid bulk temperature called Tb, and it is difficult to be directly measured in the test. As a result, a practical and precise method is needed to be developed for data processing. This paper investigates one theoretical derivation method and one practical experimental data processing method. The theoretical one is more precise but depends on much immeasurable flow field information. With some simplifications and corrections, the practical method is easy to calculate and the parameters involved are easy to measured.By dividing the channel into four kinds of segments, our methods are suitable for many kinds of channel. The energy balance method is used in the theoretical derivation method. The practical method is mainly based on local linear interpolation and the channel is divided into three parts that mainly influenced by mainstream flow, secondary flow or both two flows according to different mass flow ratios. The experimental data of the rotational turbine blade and corresponding numerical simulations show the superiority of this method in the aspect of accuracy, stability and applicability. In general, these two methods have their own characteristics and apply to different scenes, which makes them of great meaning and progressiveness.

On the measuring of film thickness profiles and local heat transfer coefficients in falling films

In this work we develop and report on two novel measurement procedures for elucidating important features of vertical falling films. The first one utilizes a laser triangulation scanner to continuously measure a film thickness profile along a 0.10 m long vertical line at a frequency of 500 Hz and a resolution of ±1.2 · 10−5 m. The second procedure is a flexible method to measure local time-averaged wall and bulk temperatures, making it possible to calculate local heat transfer coefficients at desired locations. The procedures have been implemented and evaluated in a falling film facility, under a broad range of operating conditions and covering different types of fluids and wetting rates. We have shown that the used scanner performs well for several reflecting fluids, that it resolves the flow pattern in high resolution and that it measures the film thickness with an accuracy of at least ±0.1 mm. Furthermore, we show that the measurements of the local heat transfer coefficient have an accuracy of ±0.01 K, allowing estimation of the heat transfer coefficient with an accuracy of 4–20%, depending on the operating conditions. Both procedures are accurate, robust and simple to use and their combination gives valuable insights into important features of falling films, especially how the hydrodynamics and heat transfer affect each other.

The Challenge to Measure Single-phase Convective Heat Transfer Coefficients in Microchannels

ABSTRACTThe determination of local convective heat transfer coefficients in microfluidics is a very hard task. Due to the small dimensions of channels and walls, the use of conventional measurement techniques is only partially suitable in microfluidics. For this reason, a strong effort has been made during the last decades in order to propose innovative techniques which use internal (to microdevices) sensors of reduced dimensions and/or external conventional sensors. In this paper a review of the main experimental techniques proposed for the determination of the local near-wall fluid temperature, the local wall temperature, and the local fluid bulk temperature will be given by putting in evidence for each technique's positive and negative aspects as well as their actual limitations with the aim to stimulate and address the research on this topic in the near future. The problems and the limitations existing nowadays for the accurate measurements of the local thermal properties of a convective microflow demonstrate that for the analysis of microconvection experimental data have to be always integrated by a numerical modeling of the observed system.

Experimental research in rotating wedge-shaped cooling channel with multiple non-equant holes lateral inlet

The heat transfer in a novel smooth wedge-shaped cooling channel with lateral ejection of turbine blade trailing edge is experimentally investigated in both non-rotating and rotating cases. Beside the conventional inlet at the bottom of the channel, an extra coolant injection from 8 lateral non-equant holes is introduced to improve the overall heat transfer. The total mass flow rate ratio (lateral mass flow rate/total mass flow rate) varies from 0 to 1.0. The major inlet Reynolds number and rotation number respectively vary from 10000 to 20000 and from 0 to 1.16. Experimental results show that the lateral inlet decreases local bulk temperature and increases local heat transfer at the middle and the top of the static channel. In rotating cases, the lateral inlet notably improves the heat transfer at the high-radius half channel and compensates the negative effects induced by the rotation. Both intensity and uniformity of heat transfer inside the channel are enhanced while flow resistance decreases with proper mass flow rate ratio of coolant from two inlets. The most satisfactory total mass flow rate ratio is around 2/3. This new structural style of cooling channel has huge potential and provides new direction of heat transfer of turbine blade trailing edge.

See-saw motion of thermal boundary layer under vibrations: An implication of forced piston effect

The phenomenon of piston effect is well known in supercritical fluids wherein the thermal homogenization of the bulk occurs on a very short time scale due to pressure change caused by expansion or contraction of the fluid in the thermal boundary layer. In this article, we highlight an interesting phenomenon wherein by the application of external forces (vibration) normal to the temperature gradient, see-saw motion of the thermal boundary layer is observed in weightlessness conditions. This is attributed to the thermomechanical coupling caused by the temperature change due to external forces. We term this change in the temperature field due to external forces as forced piston effect (FPE). A detailed investigation of this intriguing behavior shows that the see-saw motion is attributed to the variation of the relative thickness of the thermal boundary layer, defined on the basis of relative local bulk temperature, along the direction of vibration. This change in the temperature field, which is observed to be caused by FPE in vibration, is shown to depend on the compressibility (and thus proximity to the critical point), the imposed acceleration and the cell size. It is also found that see-saw motion persists in the presence of gravity and thus is described ubiquitous in nature for all conditions. A plot illustrating the maximum change in the temperature as a function of these parameters is further proposed.

Effects of coolant mass flow rate ratio on heat transfer in a two-inlet rotating wedge-shaped channel

Heat transfer characteristics of a novel turbine blade trailing edge internal cooling concept, two-inlet wedge-shaped channel with lateral fluid extraction, are experimentally investigated under both rotating and non-rotating conditions. In order to improve the heat transfer of the channel, a second coolant inlet is introduced in the vicinity of the low heat transfer region that previously observed in single-inlet configuration. The mass flow rate ratio (MR, second inlet mass flowrate/major inlet mass flow rate) ranges from 0 to 1. The major inlet Reynolds number and rotation number are varied from 6000 to 24,000 and 0 to 0.23, respectively. The results show that the second inlet coolant injection decreases local bulk temperature and increases local heat transfer at the high-radius half channel. In rotating cases, the second inlet notably improves the heat transfer at the top of the channel and compensates the negative effects induced by the rotation. The effect of the additional inlet is only significant in the vicinity of the coolant injection in both non-rotating and rotating cases. Moreover, with a MR of under 0.3, the second coolant injection benefits overall wall heat transfer at high rotation numbers.

Heat-flux enhancement by vapour-bubble nucleation in Rayleigh–Bénard turbulence

We report on the enhancement of turbulent convective heat transport due to vapour-bubble nucleation at the bottom plate of a cylindrical Rayleigh–Bénard sample (aspect ratio 1.00, diameter 8.8 cm) filled with liquid. Microcavities acted as nucleation sites, allowing for well-controlled bubble nucleation. Only the central part of the bottom plate with a triangular array of microcavities (etched over an area with diameter of 2.5 cm) was heated. We studied the influence of the cavity density and of the superheat $T_{b}-T_{on}$ ($T_{b}$ is the bottom-plate temperature and $T_{on}$ is the value of $T_{b}$ below which no nucleation occurred). The effective thermal conductivity, as expressed by the Nusselt number $\mathit{Nu}$, was measured as a function of the superheat by varying $T_{b}$ and keeping a fixed difference $T_{b}-T_{t}\simeq 16$ K ($T_{t}$ is the top-plate temperature). Initially $T_{b}$ was much larger than $T_{on}$ (large superheat), and the cavities vigorously nucleated vapour bubbles, resulting in two-phase flow. Reducing $T_{b}$ in steps until it was below $T_{on}$ resulted in cavity deactivation, i.e. in one-phase flow. Once all cavities were inactive, $T_{b}$ was increased again, but they did not reactivate. This led to one-phase flow for positive superheat. The heat transport of both one- and two-phase flow under nominally the same thermal forcing and degree of superheat was measured. The Nusselt number of the two-phase flow was enhanced relative to the one-phase system by an amount that increased with increasing $T_{b}$. Varying the cavity density (69, 32, 3.2, 1.2 and $0.3~\text{mm}^{-2}$) had only a small effect on the global $\mathit{Nu}$ enhancement; it was found that $\mathit{Nu}$ per active site decreased as the cavity density increased. The heat-flux enhancement of an isolated nucleating site was found to be limited by the rate at which the cavity could generate bubbles. Local bulk temperatures of one- and two-phase flows were measured at two positions along the vertical centreline. Bubbles increased the liquid temperature (compared to one-phase flow) as they rose. The increase was correlated with the heat-flux enhancement. The temperature fluctuations, as well as local thermal gradients, were reduced (relative to one-phase flow) by the vapour bubbles. Blocking the large-scale circulation around the nucleating area, as well as increasing the effective buoyancy of the two-phase flow by thermally isolating the liquid column above the heated area, increased the heat-flux enhancement.

Simulación numérica de la convección mixta en un canal vertical aletado

A numerical simulation for mixed convective airflow through a vertical finned channel was carried out by means of a MATLAB® code. Steady state and two dimensional conditions are considered for solving the problem. The discretization technique used was finite volume and the SIMPLE (Semi-Implicit Pressure Linked Equations) algorithm was used to link the pressure and velocity fields inside the computational domain. The simulation was carried out for Re = 50, 100 and 150. The heat transfer problem was solved for a temperature difference between the hot wall and the fluid temperature at the entrance of 50K and 70K. These combinations resulted on Richardson of 0.46, 1.04 and 4.16 corresponding to forced, mixed and natural convection, respectively. As a result, the fluid flow structures, the flow velocity and temperature fields as well as the recirculation zones adjacent to the fines are shown. Local Nusselt number and bulk temperature through the channel are presented. The Nusselt number and the recirculation zones increase when the Reynolds number is augmented.

One-dimensional particle simulations of Knudsen-layer effects on D-T fusion

Particle simulations are used to solve the fully nonlinear, collisional kinetic equation describing the interaction of a high-temperature, high-density, deuterium-tritium plasma with absorbing boundaries, a plasma source, and the influence of kinetic effects on fusion reaction rates. Both hydrodynamic and kinetic effects influence the end losses, and the simulations show departures of the ion velocity distributions from Maxwellian due to the reduction of the population of the highest energy ions (Knudsen-layer effects). The particle simulations show that the interplay between sources, plasma dynamics, and end losses results in temperature anisotropy, plasma cooling, and concomitant reductions in the fusion reaction rates. However, for the model problems and parameters considered, particle simulations show that Knudsen-layer modifications do not significantly affect the velocity distribution function for velocities most important in determining the fusion reaction rates, i.e., the thermal fusion reaction rates using the local densities and bulk temperatures give good estimates of the kinetic fusion reaction rates.

The Effect on the Nusselt Number of the Nonlinear Axial Temperature Distribution of Gas Flows Through Microtubes

The characteristics of nitrogen convective heat transfer through commercial stainless-steel microtubes with inner diameters of 0.172 mm and 0.750 mm are investigated both experimentally and numerically. An analysis of the total uncertainty on the Nusselt number is carried out in order to quantitatively identify the contribution introduced by the different operative parameters in the experiments. The problems related to the estimation of the correct outlet temperature measurement for gas microflows and to the determination of the axial trend of the bulk gas temperature along the microtube are deeply discussed for both microtubes tested. It is highlighted that the axial local gas bulk temperature distribution is strongly nonlinear along the tube, especially for microtubes having a small inner diameter and a thick solid wall. It has been demonstrated that the trend of the experimental Nusselt numbers as a function of the Reynolds number can be considered in good agreement with that predicted by the conventional correlations if the average bulk temperature is calculated while taking into account the axial nonlinearity of the gas bulk temperature distribution and the accurate measurement of the outlet gas temperature. The results presented in this paper give a physical explanation of the unexpected low values of the Nusselt numbers determined in the previous experimental works on gas flows through microtubes.

Determination of a Local Bulk Temperature Based Heat Transfer Coefficient for the Wetted Surfaces in a Single Inline Row Impingement Channel

High performance turbine airfoils are typically cooled with a combination of internal cooling channels and impingement. In such applications, the jets impinge against a target surface, and then exit along the channel formed by the jet plate, target plate, and side walls. Local convection coefficients are the result of both the jet impact, as well as the channel flow produced from the exiting jets. Numerous studies have explored the effects of jet array and channel configurations on both target and jet plate heat transfer coefficients. However, most current studies use the plenum temperature as the reference temperature in heat transfer calculations. This presents some difficulty to designers who need to determine heat transfer rates based on the local bulk temperatures. This paper examines three different methods to determining the local bulk temperature in a steady state impingement channel heat transfer experiment. The various methods will be compared based on their ease of application as well as their accuracy in describing the results. One method proves to be the most accurate, while another proves to be more easily implemented. The methods are compared for a single case previously studied, on a 15 hole, single row impingement channel, with dimensions of X/D=5, Y/D=4, Z/D=1 and 3, and an average jet based Reynolds number of 17,000 and 45,000. Effects due to the choice of the reference temperature in heat transfer calculations are shown to cause significant variations in the calculated heat transfer coefficients. These results point to a transition between different flow regimes in the post-impingement flow.

Convective heat transfer enhancement in low Reynolds number flows with wavy walls

The present work reports the analysis of combining low Reynolds number flows and channels with wall corrugation and the corresponding thermal exchange intensification achieved. The proposed model involves axial heat diffusion along the fluid and adiabatic regions both upstream and downstream to the corrugated heat transfer section, in light of the lower values of Reynolds numbers (and consequently Peclet numbers) that can be encountered in the present class of problems. Aimed at developing a fast and reliable methodology for optimization purposes, the related laminar velocity field is obtained by an approximate analytical solution valid for smooth corrugations and low Reynolds numbers, typical for instance of micro-channel configurations, locally satisfying the continuity equation. A hybrid numerical-analytical solution methodology for the energy equation is proposed, based on the Generalized Integral Transform Technique (GITT) in partial transformation mode for a transient formulation. The hybrid approach is first demonstrated for the case of a smooth parallel-plates channel situation, and the importance of axial heat conduction along the fluid is then illustrated. Heat transfer enhancement is analyzed in terms of the local Nusselt number and dimensionless bulk temperature along the heat transfer section. An illustrative sinusoidal corrugation shape is adopted and the influence of Reynolds number and corrugation geometric parameters is then discussed.