Andallib Tariq

Estimation of convective heat transfer coefficient from transient liquid crystal data using an inverse technique

An estimation of the local Nusselt number distribution for a flat and a ribbed surface from transient liquid crystal images is presented. Liquid crystal thermography generates color images of the time-varying surface temperature field, when an initially heated surface is subjected to cooling in forced flow. The inverse technique compares the approximate numerical solution with the transient experimental temperature distribution, and enforces the applicable physical laws in such a way that a globally correct Nusselt number distribution is predicted. The related optimization problem has been solved by a conjugate gradient method, with a stabilization scheme based on additional experimental data. The partial differential equations arising at the intermediate stages have been solved numerically using the finite difference technique. Predictions of the local Nusselt number have been compared with the full numerical solution based on unsteady incompressible laminar flow, as well as the one-dimensional semi-infinite solid approximation applied to experimental data. Reynolds numbers considered in the study are 160 and 260, based on the rib height. Results show that the inverse technique is capable of resolving sharp as well as gradual changes in the heat transfer rates for the flat plate and the rib geometries. The peak in the Nusselt number distribution for flow past a rib is seen to fall at a location where the flow reattaches with the flat surface. The inverse technique is robust with respect to signal length, and within limits it is insensitive to noise in the experimental data.

Liquid Crystal Heat Transfer Measurements in a Rectangular Channel with Solid and Slit Rib

AbsractAn experimental investigation on the heat transfer effectiveness of solid and slit ribs mounted on the bottom surface of a rectangular channel has been carried out at Reynolds numbers of 13400, 22600, 32100 and 40800. The rib height to hydraulic diameter ratio (e/Dh)set during experiment is equal to 0.0624. The surface Nusselt number results from transient liquid crystal thermography are presented. The heat transfer enhancement performance analysis has been carried out using entropy generation principle. The slit rib is superior to solid rib from both heat transfer augmentation and pressure penalty point of view. The performance of the slit rib is a function of the open area ratio (β) and the location of the slit (b) from the bottom test surface. The optimum open area ratio is 20% and the slit located symmetrically from the top and bottom surface of the rib is the optimum location of the slit. The heat transfer augmentation of the slit rib (β=20%) is 61% in comparison to 40% for the solid rib at Re=32100 and the pressure penalty for the slit rib is 7% lower than the solid rib. The entropy generation for the slit rib is 33% less than that of the solid rib.

DETAILED MEASUREMENT OF HEAT TRANSFER AND FLOW CHARACTERISTICS IN RECTANGULAR DUCT WITH RIB TURBULATORS MOUNTED ON THE BOTTOM SURFACE

ABSTRACT The present work is an experimental study to investigate the heat transfer and flow characteristics in the entrance region of a rectangular channel with a single and two solid square ribs mounted on the bottom surface of the channel. Hot wire anemometry (HWA) and resistance thermometry (RTD) have been utilized for the velocity and temperature measurements in the flow-field. Liquid crystal thermography (LCT) has been employed to map the surface temperature profiles and evaluation of the heat transfer coefficient. The effectiveness of one-rib and two-ribs is studied from flow modulation and heat transfer enhancement point of view. The Reynolds number (based on the hydraulic diameter of the channel) is set equal to 2.09 × 104. The rib pitch-to-height ratio set during the experiment is equal to 10. The Nusselt number variation on the bottom surface for the no-rib case is similar to what is observed from the law of the wall. The Nusselt number augmentation results from transient LCT closely follows the energy balance check. The turbulent boundary layer well past the ribs for both one-rib and two-rib case is different from that of the plane surface as evident from the higher Nusselt number observed in the far-field region. The stagnation region around the second rib is not as significant as that around the first rib. There is some degree of correlation between the velocity fluctuation and temperature fluctuation, the former being significant in the core of the shear layer while later is significant close to the wall. Overall, the thermal boundary layer, the hydrodynamic boundary layer, flow visualization and the Nusselt number results are observed to correlate with each other very well.

Detailed Heat Transfer Investigation Inside a Rectangular Duct with an Array of Ventilated Rib Turbulators

Rib turbulators are extensively studied turbulence promoters employed in numerous industrial applications such as cooling of gas turbine blades and different electronic components, nuclear reactors and compact heat exchangers etc. for heat transfer augmentation. Some of the recent studies have shown the effect of altering rib profiles other than the square along with the effect of slit within it at fixed ratio of rib pitch to height (p/e). The present investigation focuses to explore the effects of p/e (p/e = 6, 8, 10, 12) on heat transfer augmentation and pressure penalty. Liquid crystal thermography (LCT) was applied to measure surface temperature distribution with temporal variation and ultimately the local heat transfer coefficients. Transient experiments have been conducted in an open-circuit, suction type air flow system at Reynolds number (Re) ranging from 9400 to 58850 (based on hydraulic diameter of the test section) and at a fixed value of rib height to channel hydraulic diameter ratio (e/D h ) of 0.125. The importance of current work is to appraise the potential impact of rib geometry especially the p/e ratio on the heat transfer enhancement corresponds to square solid and continuous converging slit rib configurations. It is expected, that the experimental data generated from this study will broaden the understanding of heat transfer from fundamental perspective and will serve as a benchmark dataset for validation of computational models.

Correlations of Thermal Contact Conductance for Nominally Flat Metallic Contact in Vacuum

Heat transfer experiments were performed in a vacuum environment (0.045 torr) to find out solid spot contact conductance for nominally flat surfaces of copper, brass, and stainless steel with different surface roughnesses (1–5 μm) for each specimen under several load conditions (0.6–15 MPa). A precise estimation of thermal contact conductance for the interface of sets of similar materials has been carried out employing a steady-state approach. Results have been compared with the theoretical models based on plastic and elastic deformation based theories for nominally flat metallic contacts. It has been demonstrated that a dire need exists to develop an experimentally based generalized correlation of thermal contact conductance for nominally flat metallic contacts that can be used by design engineers as well as for research purposes in different fields. Along with the generalized correlation, a specific correlation for thermal contact conductance has been proposed for three sets of materials. The thermal contact conductance of each set of materials presented in the form of specific correlations for different roughness parameters is one of the most important outcomes of the present research.

Experimental Investigation of Heat Transfer Enhancement in Rectangular Duct with Pentagonal Ribs

ABSTRACT Rib turbulators are extensively used in augmentation of convective heat transfer in several applications related to heat exchange and cooling in thermal energy systems. Present experimental investigation examines the local heat transfer and friction factor characteristics of pentagonal ribs mounted on bottom heated wall of a rectangular channel. The emphasis is towards assessing and analysing the potential impact of varying chamfering angle (0 to 20°) and rib pitch to height ratio (6 to 12) on the overall heat transfer enhancement and its distribution on the surface. Experiments are performed at different Reynolds numbers ranging from 9400 to 58850. Liquid crystal thermography is applied to measure surface temperature distribution and finally to demonstrate the local heat transfer coefficient over the ribbed surface. The results depict that the local augmentation Nusselt number distribution is axisymmetric and shows 2-dimensionalty in heat transfer distribution. Pentagonal ribs show a significant improvement for the low heat transfer zones in leeward vicinity of the square rib, specially prominent at higher Reynolds number, and therefore seen as the potential benefit in terms of obviating the hotspots. It is observed that the pentagonal ribs lead to superior heat transfer enhancement in conjunction with significant reduction in pressure penalty as compared to square ribs and thus ensures an enhanced thermo-hydraulic performance.

Detailed Investigation on Rib Turbulated Flow Inside a Rectangular Duct

Rib turbulators are the most intensively studied passive technique, which promotes near wall turbulence in the internal cooling passages of heat transfer devices. However, there exists the tradeoff between the pressure penalty and heat transfer enhancement. For suggesting a correct rib configuration for particular application, it is necessary to understand the flow mechanism behind rib tabulators. For this purpose an experimental setup has been designed to investigate the detailed flow field and corresponding effect on heat transfer characteristics using Particle Image Velocimetry and Liquid Crystal Thermography respectively. In the literature the detailed flow field investigation as well as the thermal characterizations behind the rib other than rectangular/square cross sectional shape is found to be limited. The present work is an experimental investigation inside a rectangular duct for flow behind the trapezoidal type of rib with changing angle at different Reynolds numbers. The emphasis is towards assessing the potential impact of varying chamfering angle over the flow structures and its subsequent effect on heat transfer enhancement as well as in obviating the hot spots in the vicinity behind the chamfered rib turbulators.Copyright

LCT and PIV Investigations Behind Trapezoidal-Rib With a Slit Mounted on Bottom Wall of a Rectangular Duct

Various rib turbulator geometries have been used earlier to investigate the heat transfer and fluid flow characteristics owing to its vast industrial applications. Permeable ribs are reported to provide better performance in terms of heat transfer enhancement and pressure penalty. The trapezoidal rib with variable downstream chamfering and a centrally placed longitudinal slit has been used for the detailed flow field and heat transfer investigations using particle image velocimetry and liquid crystal thermography. Objective of the present work is to study the combined effect of rib chamfering along with a continuous slit and establish an understanding of the underlying flow mechanism and the corresponding heat transfer distribution. Experiments were carried out for hydraulic diameter based Reynolds numbers of 61480 in a rectangular duct for flow over rib with 12.5% blockage ratio and 25% open area ratio. The chamfer angles were varied from 0 to 20 degree. It has been observed that the slitted rib cause shorter reattachment length and reduced pressure penalty as compared to its solid counterpart.Copyright

Heat Transfer and Flow Characteristics of a Rib With a Slit

The present investigation is an experimental study of convective heat transfer in the entrance region of a rectangular channel with a single surface mounted slit rib. The open area ratios of the slit rib set during the experiment are equal to 10, 20, 30, 40 and 50%. Hotwire anemometry (HWA) and resistance thermometry (RTD) have been used for velocity and temperature measurement respectively. Both mean and turbulent statistics of the velocity and temperature fluctuations have been reported. Smoke visualization has also been carried out to obtain a qualitative picture of the flow field behind the rib. The surface Nusselt number has been determined from liquid crystal thermography (LCT). The Reynolds number based on the hydraulic diameter of the channel has been set at Re = 32,100. The nature of the flow through the slit and its interaction with the shear layer from the top of the rib depend on the size of the slit. For the slit rib with higher open area ratio (β = 40 and 50%), the bottom part of the slit rib behaves like an independent small rib with its own reattachment region. At smaller open area ratio (β = 10, 20 and 30%), the flow through the slit manipulates the reattaching shear layer from the top of the rib. The size of the slit and its location from the bottom channel surface are the primary parameters responsible for the modification and manipulation of the flow behavior of a slit rib in comparison to the solid rib.Copyright

Detailed heat transfer and friction factor characteristics in a rectangular duct with alternate solid and converging-slit ribs

ABSTRACT Liquid crystal thermography and pressure drop measurements have been carried out to study the heat transfer and frictional characteristics in a rectangular duct with solid ribs (C1), converging slit-ribs (C2), and alternate solid-slit ribs (C3) mounted transversely on the bottom wall, where C2 carries a continuous converging-slit in the flow direction. Effect of rib configurations, and rib pitch to height ratios (6, 8, 10, and 12) has been investigated at Re of 9400, 26160, 42500, and 58850. Results show that converging-slit considerably enhances the heat transfer rate in the downstream vicinity, and help in obviating the local hot spot formation. Abbreviations: LCT: Liquid crystal thermography; HTC: Heat transfer coefficient; LHI: Laser holographic interferometry; NST: Naphthalene sublimation technique; IR: Infrared; TPF: Thermo-hydraulic performance; PIV: Particle image velocimetry.