Local Wall Heat Transfer Coefficient Research Articles

Experimental and numerical investigations on the local wall heat transfer coefficient in a narrow packed bed with spheres

Packed beds are used at the laboratory scales to model and understand the phenomena of water flow through soil and rocks. The behaviour of local wall heat transfer coefficient with randomised packing of mono-dispersed spheres is examined in the present study. Under steady-state conditions, the high-resolution local wall temperature data is obtained by placing the infrared (IR) camera in close proximity to the test section. The local wall heat transfer coefficient is computed using the measured bulk fluid temperature, local wall temperature data and applied heat flux. Physical and numerical experiments were conducted for the narrow bed to particle diameter ratio of 1.25 using random packing of mono-dispersed (impermeable) glass spheres (diameter 6 mm) in order to be able to recreate similar geometry for numerical simulations of fluid flow and heat transfer. An attempt is made for recreation of the experimental narrow bed geometry by using the technique of method of gaps and computational fluid dynamics simulations were performed. The results of simulations are presented in appropriate velocity vectorial plots and flow stream lines revealing complex 3D mixing patterns inside void spaces of the narrow bed. Local variations of wall heat transfer coefficient are found along the packed bed and the numerical simulations of fluid flow and heat transfer reveal interesting complex flow patterns inside the bed.

Local wall heat transfer coefficient measurement in a packed bed of cylinders using infrared (IR) thermography technique and application of random packing of cylindrical particles inside concentric tube heat exchangers with water as working medium

Heat transfer enhancement is challenging area in which different techniques are utilized to dissipate the generated heat during operation. Packed beds are devices which enhance heat transport while being compact and are used in several applications such as energy storage, heat exchange devices, catalysis, food processing etc. In the present study, the behavior of wall heat transfer coefficient with randomized packing of equal aspect ratio cylinders is investigated. In this work, the local wall heat transfer coefficient is calculated from local wall temperature data obtained using infrared (IR) thermography in packed beds with randomized packing of cylinders under steady state conditions with water as the working fluid. The randomized packing of cylinders is done inside a concentric tube heat exchanger and the heat transfer enhancement is studied. Experiments are conducted for bed to equivalent particle diameter ratio 2 using random packing of mono-dispersed glass cylinders (dcyl = 6 mm and lcyl = 6 mm). The local wall temperatures are measured using an infrared (IR) camera while the fluid flow takes place through the packed beds. In literature, it is found that wall region contribution to heat transfer is above 90%. The comparison of enhancement in heat transfer for both packing of spheres and cylinders inside concentric tube heat exchangers is done and it is observed that the random sphere packing is superior to cylinder packing.

Measurement of local wall heat transfer coefficient in randomly packed beds of uniform sized spheres using infrared thermography (IR) and water as working medium

The needs for compact and effective heat transfer equipments are in growing demand and porous media caters to this need. Packed beds are a simple type of porous media devices which help in transport of mass, momentum and heat. The applications of packed beds are in chemical reactors, distillation, catalysis, thermal energy storage, food processing, solar power plants and several other areas. They act as enhancement devices for different transport processes. In the present study, the behavior of wall heat transfer coefficient with randomized packing of uniformly sized spheres is investigated.In the present work, the local wall heat transfer coefficient is estimated from local wall temperature data obtained using infrared (IR) thermography for the packed beds with randomized packing of uniformly sized spheres under steady state conditions with water as the working fluid. The experiments are conducted for bed to particle diameter ratios of 6, 3 and 2 using random packing of mono-dispersed glass spheres. The two-layer model theory for heat transfer in packed beds is examined in terms of contribution or core zone and wall zone in packed beds. The comparison of heat transfer enhancement aspects with different insert configurations is done and packed beds are found effective.

A Network-of-Voids Model to Assess Wall Flow Patterns and Heat Transfer for Low Aspect Ratio Packed-Bed Reactors

Exothermic packed bed catalytic reactors are usually characterised by a low diameter-aspect ratio to facilitate heat transfer. In operation, these reactors often exhibit localized regions with much higher temperatures referred to as hot spots. A new model based on a 2-D network-of voids (NoV) has been devised to explore wall heat transfer behaviour for such low aspect ratio packed tubes. Random placement of (packing) particles is used to provide a simple NoV framework for implicitly creating the tortuous fluid flows amongst the resulting randomized inter-connecting voids. This is a computationally tractable strategy for exploring the haphazard appearance of individual tube pin-hole burn-outs amongst the typically thousands, or tens of thousands, of tubes within high temperature industrial multi-tubular configurations. Although presently limited to 2-D, the model captures many natural features of the flow and heat transfer of randomly packed tubes, especially huge variations in wall and cross flows and consequently massive variations in local wall heat transfer coefficients along the length of individual tubes. The model is potentially superior to those based upon averaged properties, which do not properly distinguish the solid and fluid phases. The network-of-voids concept is readily extended to 3-D, in order to achieve geometric congruence of the model and assemblies of individual particles.

Wall Heat Transfer in Stirred Tank Reactors

Local wall heat-transfer coefficients have been measured for water and for the non-Newtonian systems consisting of 0.3, 0.6, and 1.0 wt % carboxymethylcellulose in water. The results were compared to existing correlations, and good agreement was found for the water system outside the impeller region. In the impeller region, deviations were large. For the non-Newtonian systems, a functional dependence between the Reynolds number and the heat-transfer coefficient, given by Nu/(Pr0.33Vi0.14) ∝ Re0.8, was found to fit the data well. Support for this was found by a closer examination of the existing literature. This is in contrast to the currently accepted dependence on a Reynolds number exponent of 0.67−0.68. On the basis of the data, a new correlation for non-Newtonian fluids has been suggested. Radial temperature profiles for the wall region were also measured and presented.

Heat transfer measurements and simulation of KATAPAK-M® catalyst supports

The heat transfer from a heated wall to a cold fluid in a tube filled with KATAPAK-M elements is investigated. The overall heat transfer coefficient, the local wall heat transfer coefficient and the effective radial conductivity are determined from the experimental results using the pseudohomogeneous plug flow model. The same heat transfer is also simulated with three-dimensional numerical flow simulations (CFD). The simulation results agree well with the experimental findings and reproduce the characteristic flat radial temperature profiles. The overall heat transfer coefficients obtained by numerical simulations support the experimental results. It is shown that the local wall heat transfer coefficient and the effective radial conductivity depend strongly on the experimental resolution of the temperature profiles close to the tube wall since the pseudohomogeneous model does not adequately represent the relatively flat temperature profiles typical for KATAPAK-M. With the numerical results presented in this paper, a first reliable basis for a better understanding of catalytic reactors using KATAPAK-M is created.

Forced Convective Heat Transfer in Curved Diffusers

Measurements of the velocity characteristics of the flows in two curved diffusers of rectangular cross section with C and S-shaped centerlines are presented and related to measurements of wall heat transfer coefficients along the heated flat walls of the ducts. The velocity results were obtained by laser-Doppler anemometry in a water tunnel and the heat transfer results by liquid crystal thermography in a wind tunnel. The thermographic technique allowed the rapid and inexpensive measurement of wall heat transfer coefficents along flat walls of arbitrary boundary shapes with an accuracy of about 5 percent. The results show that an increase in secondary flow velocities near the heated wall causes an increase in the local wall heat transfer coefficient, and quantify the variation for maximum secondary-flow velocities in a range from 1.5 to 17 percent of the bulk flow velocity.

Heat Transfer and Temperature Field Experiments in a Cavity With Rotation, Recirculation, and Coolant Throughflow

Local wall heat transfer coefficients and fluid temperature distributions were measured in a cavity consisting of a pair of parallel disks and a cylindrical shroud. One of the disks was rotating, whereas the other disk and the shroud were stationary. Coolant air entered the cavity through a central aperture in the rotating disk and exited through an annular gap at the rim of the rotating disk. The coolant flow rate, the disk rotational speed, and the cavity aspect ratio (disk separation distance to radius) were varied throughout the course of the experiments. The latter parameter took on values as large as two. The heat transfer results and the fluid isotherm maps suggested that the flow pattern within the cavity was markedly different depending upon whether the coolant stream or the pumping action of the rotating disk was predominant. The surface distributions of the heat transfer coefficients reversed the direction of their spatial variation over the range from no rotation to high rotation. However, the maximum values of the Nusselt number curves for no rotation were as high as the maximum values of the curves for corresponding cases with high rotation. The isotherm maps for the no-rotation cases revealed that the major portion of the cavity was filled with nearly isothermal fluid. On the other hand, in the presence of strong rotation, there were substantial fluid temperature variations throughout the cavity.