Heat Transfer in Porous Graphite Foams

2013 ◽  
Vol 136 (3) ◽  
Author(s):  
Brian E. Thompson
Keyword(s):  
Heat Transfer ◽  
Thermal Performance ◽  
Heat Exchangers ◽  
Pressure Loss ◽  
Average Diameter ◽  
Cross Sectional ◽  
Pressure Losses ◽  
Porous Graphite ◽  
Order Of Magnitude ◽  
Measured Pressure

Measured values of heat transfer and pressure loss are presented for a variety of porous graphite foams in subsonic turbulent airflow. These foams were developed over the last decade to find combinations of high conductivity, porosity, strength, and low density suitable for application to rapid cooling of electronics and to corrosionless heat-exchangers. Measured maxima in the thermal performance that is the ratio of heat transfer to pressure loss, were correlated the pore structure obtained from scanning electron microscopy, to show a linear dependence of thermal performance on the average diameter of interpore windows representative of the cross-sectional area through which cooling air flows. For the same heat transfer, measured pressure losses were reduced by over two orders of magnitude by increasing pore and window diameters. However, the best thermal performance of porous graphite foams that were strong enough for industrial application, had measured pressure losses that were more than an order of magnitude greater than losses in conventional finned heat exchangers.

Optimization of Plate Heat Exchangers with Intermittent Ridges

2015 ◽  
Vol 752-753 ◽  
pp. 820-827 ◽  
Author(s):  
Vaclav Dvorak
Keyword(s):  
Heat Transfer ◽  
Heat Exchanger ◽  
Objective Function ◽  
Heat Exchangers ◽  
Pressure Loss ◽  
Optimization Method ◽  
Plate Heat Exchanger ◽  
Plate Heat ◽  
Pressure Losses ◽  
Computational Mesh

Research of devices for heat recovery is currently focused on increasing the temperature and heat efficiency of plate heat exchangers. The goal of optimization is not only to increase the heat transfer or even moisture but also reduce the pressure loss and possibly material costs. This study deals with a plate heat exchanger with wall shaped by intermittent ridges. We used software fluent and user defined deforming to deform computational mesh and create various heat exchange walls with different number of ridges and different number of set-offs. The intention of the set-offs is to discompose boundary layer inside channels created by ridges, mix the temperature field and thus intensify the heat transfer. We used previously formulated objective function, which is a linear combination of efficiency and pressure loss, and a simple local method to optimize the heat exchanger for required pressure loss. It was found that the objective function surface is monotone and unimodal, but is not smooth. The global optimums were identified and it was shown that the optimal wall shape has no set-off for low pressure losses. The optimal count of ridges and optimal count of set-offs rise with higher required pressure loss. It was proved that the suggested objective function is suitable for optimization of a counterflow plate heat exchanger, but use of a global optimization method would be beneficial.

scholarly journals Experimental implementation of thermal enhancement performance of air heat exchanger’s pipes utilizing unconventional turbulator

2021 ◽  
pp. 35-44
Author(s):  
Ali Abdulwahab Ismaeel ◽  
Nassr Fadhil Hussein ◽  
Kadhim H. Suffer ◽  
Zuradzman M Razlan
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Rate ◽  
Thermal Performance ◽  
Heat Exchangers ◽  
Transfer Rate ◽  
Heated Surface ◽  
Constant Heat Flux ◽  
Working Fluid ◽  
Practical Implementation ◽  
Cross Sectional

Heat exchangers are widely used in industry, however, raising their performance are important for the variety of applications. Consequently, efficiency improvement associated with low production cost is considered in this experimental work. The current study aims to enhance the rate of heat transfer in pipe-type heat exchangers experimentally by using a novel nozzle as a turbulator. The cross-sectional shape of the nozzle is hexagonal, and the diameter ratio DR is equal to 0.5. Constant heat flux was maintained in the vicinity of the section of the test tube, while the working fluid was pumped into the open system at six discrete Reynolds number values ranging from 6000 to 19500. To investigate the effect of distance among the pieces, three turbulators with different numbers were assigned and named as (N=4, 5 and 6). The results indicated an increase of 172 %, 194 % and 216 % of the heat transfer rate for cases 4, 5 and 6 respectively comparing to the benchmark tube. On the other hand, the friction factor values increased remarkably due to the inserting of turbulators by about of 722.9 % for N=4, 823.9 % for N=5 and 886.7 % for N=6 compared to a plain tube case. Moreover, it has been established that with the insertion of 6 pieces two enhancements was observed; heat transfer rate and thermal performance, where, thermal performance of all cases exceeds unity (maximum thermal performance of 1.62 has been obtained by inserting 6 pieces of hexagonal nozzles turbulators). A comparison with another types of vortex generators shows the gap between the turbulator and heated surface offers a solution for problems occurred in the pipes of heat exchanger. The study therefore suggests a wider practical implementation of the turbulators

An Experimental and Numerical Study of Heat Transfer and Pressure Losses of V- and W-Shaped Ribs at High Reynolds Numbers

2007 ◽  
Author(s):  
Michael Maurer ◽  
Jens von Wolfersdorf ◽  
Michael Gritsch
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Enhancement ◽  
Thermal Performance ◽  
Pressure Loss ◽  
Numerical Study ◽  
Reynolds Numbers ◽  
Transfer Enhancement ◽  
Height Ratio ◽  
Reduced Pressure ◽  
Pressure Losses

An experimental and numerical investigation was conducted to assess the thermal performance of V- and W-shaped ribs in a rectangular channel. The ribs were located on one channel sidewall in order to simulate a typical combustor liner cooling. The cross section of the channel had an aspect ratio of 2:1. Local heat transfer coefficients were measured using the transient thermochromic liquid crystal technique. Pressure taps along the channel sidewall were used to obtain the periodic pressure losses. The rib height-to-hydraulic diameter ratio (e/Dh) was set to 0.02, and the rib pitch-to-height ratios (P/e) were 5 and 10. The Reynolds numbers investigated varied from 80,000 to 500,000. All rib configurations were additionally investigated numerically and the obtained computational results were compared with experimental data. For all computations the commercial software FLUENT™ was used with a two-layer k-ε turbulence model. It could be demonstrated that applying W-shaped ribs instead of V-shaped ribs has the advantage of an increased heat transfer enhancement, but is accompanied by a rise in pressure loss. Reducing the rib pitch-to-height ratio from 10 to 5 decreases the heat transfer enhancement, but results in a significantly reduced pressure loss. Finally, the best thermal performance was found for W-shaped ribs with a pitch-to-height ratio of 10, having a slightly increased pressure loss but with considerable rise in heat transfer enhancement compared to V-shaped ribs.

Heat Transfer and Pressure Losses in a Narrow Dimpled Channel Structured With Spherical Protrusions

2006 ◽  
Author(s):  
I. Borisov ◽  
A. Khalatov ◽  
S. Kobzar ◽  
B. Glezer
Keyword(s):  
Heat Transfer ◽  
Gas Turbine ◽  
Heat Exchangers ◽  
Pressure Loss ◽  
Reynolds Numbers ◽  
Low Pressure ◽  
Reynolds Analogy ◽  
Narrow Channels ◽  
Pressure Losses ◽  
Spherical Protrusions

Achieving a high heat transfer effectiveness at low pressure losses in narrow channels continue to present a significant challenge for designers of gas turbine components and heat exchangers. The task of low pressure losses often requires that some of these components, particularly heat exchangers, have to operate at a flow rate that corresponds to relatively low Reynolds numbers ranging from 200 to 800. The operation at higher Reynolds numbers permits to improve the recuperator performance, however it leads to unacceptable pressure losses. Introduction of hemispherical dimples for heat transfer augmentation has become recently one of the promising techniques for achieving higher heat transfer performance in narrow channels at an acceptable pressure loss level. A structural support between the primary heat transfer surfaces is usually required for a double walled back-side cooled turbine components and multichannel heat exchangers (recuperators), operating with pressure differential between cold and hot channels. For this purpose in the current study spherical protrusions (reversed dimples) were employed on a panel opposing a dimpled panel. This structural arrangement was expected to result in additional blockage of the channel cross-section and certain increases in a pressure loss. The experimental study has been performed to assess the effect of spherical dimples and protrusions on heat transfer and pressure losses in a formed narrow channel. The airflows in the experiments corresponded to the Reynolds number ranging from 800 to 6,500. A dimple diameter and depth were 10.0 mm and 2.0 mm, correspondingly; the protrusions established the 2.0 mm height of the channels. Both the in-line and staggered dimple arrangements were studied with the x-pitch ranging from 9.0 to 18.0 mm and z-pitch changing from 13.0 to 18.0 mm. The data presented in this paper include results for measurements of average heat transfer coefficients and pressure losses. Reynolds analogy factor and thermal performance of the primary surface were obtained and discussed in the paper. Considering potential application of studied surfaces for gas turbine heat exchangers, the paper provided a comparison between a “pure” dimpled channel, dimpled channel with protrusions against a more traditional channel with sinusoidal corrugated primary surface. As expected, the protrusions in the channel enhanced the heat transfer, but led to increased pressure losses due to the partial destruction of the dimple-generated vortex structures. Nevertheless, it was demonstrated that the Reynolds analogy factor of 0.4 could be achieved in a dimpled channel with protrusions, resulting in overall pressure losses of under 5% for the application in a recuperator core.

An Experimental and Numerical Study of Heat Transfer and Pressure Loss in a Rectangular Channel With V-Shaped Ribs

2006 ◽  
Author(s):  
Michael Maurer ◽  
Jens von Wolfersdorf ◽  
Michael Gritsch
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Enhancement ◽  
Thermal Performance ◽  
Pressure Loss ◽  
Numerical Study ◽  
Rectangular Channel ◽  
Reynolds Numbers ◽  
Transfer Enhancement ◽  
High Reynolds Numbers ◽  
High Reynolds

An experimental and numerical study was conducted to determine the thermal performance of V-shaped ribs in a rectangular channel with an aspect ratio of 2:1. Local heat transfer coefficients were measured using the steady state thermochromic liquid crystal technique. Periodic pressure losses were obtained with pressure taps along the smooth channel sidewall. Reynolds numbers from 95,000 to 500,000 were investigated with V-shaped ribs located on one side or on both sides of the test channel. The rib height-to-hydraulic diameter ratios (e/Dh) were 0.0625 and 0.02, and the rib pitch-to-height ratio (P/e) was 10. In addition, all test cases were investigated numerically. The commercial software FLUENT™ was used with a two-layer k-ε turbulence model. Numerically and experimentally obtained data were compared. It was determined that the heat transfer enhancement based on the heat transfer of a smooth wall levels off for Reynolds numbers over 200,000. The introduction of a second ribbed sidewall slightly increased the heat transfer enhancement whereas the pressure penalty was approximately doubled. Diminishing the rib height at high Reynolds numbers had the disadvantage of a slightly decreased heat transfer enhancement, but benefits in a significantly reduced pressure loss. At high Reynolds numbers small-scale ribs in a one-sided ribbed channel were shown to have the best thermal performance.

scholarly journals Efficiency Improvement of Miniaturized Heat Exchangers

Fluids ◽  
2021 ◽  
Vol 6 (1) ◽  
pp. 25
Author(s):  
Iris Gerken ◽  
Thomas Wetzel ◽  
Jürgen J. Brandner
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Enhancement ◽  
Heat Exchangers ◽  
Assessment Method ◽  
Flow Path ◽  
Transfer Enhancement ◽  
Pressure Losses ◽  
Pin Fin ◽  
Additional Pressure ◽  
The Impact

Micro heat exchangers have been revealed to be efficient devices for improved heat transfer due to short heat transfer distances and increased surface-to-volume ratios. Further augmentation of the heat transfer behaviour within microstructured devices can be achieved with heat transfer enhancement techniques, and more precisely for this study, with passive enhancement techniques. Pin fin geometries influence the flow path and, therefore, were chosen as the option for further improvement of the heat transfer performance. The augmentation of heat transfer with micro heat exchangers was performed with the consideration of an improved heat transfer behaviour, and with additional pressure losses due to the change of flow path (pin fin geometries). To capture the impact of the heat transfer, as well as the impact of additional pressure losses, an assessment method should be considered. The overall exergy loss method can be applied to micro heat exchangers, and serves as a simple assessment for characterization. Experimental investigations with micro heat exchanger structures were performed to evaluate the assessment method and its importance. The heat transfer enhancement was experimentally investigated with microstructured pin fin geometries to understand the impact on pressure loss behaviour with air.

Numerical Prediction of Flow and Heat Transfer in a Parallel Plate Channel With Staggered Fins

1987 ◽  
Vol 109 (1) ◽  
pp. 25-30 ◽  
Author(s):  
K. M. Kelkar ◽  
S. V. Patankar
Keyword(s):  
Heat Transfer ◽  
Local Heat Transfer ◽  
Periodic Variation ◽  
Local Heat ◽  
Temperature Fields ◽  
Parallel Plates ◽  
Constant Property ◽  
Cross Sectional ◽  
Flow And Heat Transfer ◽  
Order Of Magnitude

Fluid flow and heat transfer in two-dimensional finned passages were analyzed for constant property laminar flow. The passage is formed by two parallel plates to which fins are attached in a staggered fashion. Both the plates are maintained at a constant temperature. Streamwise periodic variation of the cross-sectional area causes the flow and temperature fields to repeat periodically after a certain developing length. Computations were performed for different values of the Reynolds number, the Prandtl number, geometric parameters, and the fin-conductance parameter. The fins were found to cause the flow to deflect significantly and impinge upon the opposite wall so as to increase the heat transfer significantly. However, the associated increase in pressure drop was an order of magnitude higher than the increase in heat transfer. Streamline patterns and local heat transfer results are presented in addition to the overall results.

Heat Transfer and Pressure Loss Characteristics of Pin-Fins With Different Shapes in a Wide Channel

2017 ◽  
Author(s):  
Jin Xu ◽  
Jiaxu Yao ◽  
Pengfei Su ◽  
Jiang Lei ◽  
Junmei Wu ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Nusselt Number ◽  
Thermal Performance ◽  
Pressure Loss ◽  
Bottom Surface ◽  
Number Range ◽  
Pin Fin ◽  
Nominal Diameter ◽  
Pin Fins

Convective heat transfer enhancement and pressure loss characteristics in a wide rectangular channel (AR = 4) with staggered pin fin arrays are investigated experimentally. Six sets of pin fins with the same nominal diameter (Dn = 8mm) are tested, including: Circular, Elliptic, Oblong, Dropform, NACA and Lancet. The relative spanwise pitch (S/Dn = 2) and streamwise pitch (X/Dn = 4.5) are kept the same for all six sets. Same nominal diameter and arrangement guarantee the same blockage area in the channel for each set. Reynolds number based on channel hydraulic diameter is from 10000 to 70000 with an increment of 10000. Using thermochromic liquid crystal (R40C20W), heat transfer coefficients on bottom surface of the channel are achieved. The obtained friction factor, Nusselt number and overall thermal performance are compared with the previously published data from other groups. The averaged Nusselt number of Circular pin fins is the largest in these six pin fins under different Re. Though Elliptic has a moderate level of Nusselt number, its pressure loss is next to the lowest. Elliptic pin fins have pretty good overall thermal performance in the tested Reynolds number range. When Re>40000, Lancet has a same level of performance as Circular, but its pressure loss is much lower than Circular. These two types are both promising alternative configuration to Circular pin fin used in gas turbine blade.

scholarly journals Execution of a Smart Prediction Tool to Evaluate Thermal Performance in a heat exchanger by using Single Elliptical Leaf Strips with altered Angle

2019 ◽  
Vol 8 (11) ◽  
pp. 123-131
Keyword(s):  
Heat Transfer ◽  
Heat Exchanger ◽  
Thermal Performance ◽  
Heat Exchangers ◽  
Industrial Applications ◽  
Generalized Regression Neural Network ◽  
Prediction Tool ◽  
Pressure Drops ◽  
Transfer Rates ◽  
Mass Flow Rates

Heat exchangers are prominent industrial applications where engineering science of heat transfer and Mass transfer occurs. It is a contrivance where transfer of energy occurs to get output in the form of energy transfer. This paper aims at finding a solution to improve the thermal performance in a heat exchanger by using passive method techniques. This experimental and numerical analysis deals with finding the temperature outlets of cold and hot fluid for different mass flow rates and also pressure drop in the tube and the annular side by adding an elliptical leaf strip in the pipe at various angles. The single elliptical leaf used in experiment has major to minor axes ratios as 2:1 and distance of 50 mm between two leaves are arranged at different angular orientations from 0 0 to 1800 with 100 intervals. Since it’s not possible to find the heat transfer rates and pressure drops at every orientation of elliptical leaf so a generalized regression neural network (GRNN) prediction tool is used to get outputs with given inputs to avoid experimentation. GRNN is a statistical method of determining the relationship between dependent and independent variables. The values obtained from experimentation and GRNN nearly had precise values to each other. This analysis is a small step in regard with encomiastic approach for enhancement in performance of heat exchangers

scholarly journals Performance Analysis and Design Optimization of Shell and Tube Heat Exchangers

2021 ◽  
Author(s):  
praveen math
Keyword(s):  
Heat Transfer ◽  
Heat Exchanger ◽  
Heat Exchangers ◽  
Pressure Loss ◽  
Fluid Flows ◽  
Hot Water ◽  
Flow Conditions ◽  
Shell Side ◽  
Shell And Tube ◽  
Side Flow

Abstract Shell and Tube heat exchangers are having special importance in boilers, oil coolers, condensers, pre-heaters. They are also widely used in process applications as well as the refrigeration and air conditioning industry. The robustness and medium weighted shape of Shell and Tube heat exchangers make them well suited for high pressure operations. The aim of this study is to experiment, validate and to provide design suggestion to optimize the shell and tube heat exchanger (STHE). The heat exchanger is made of acrylic material with 2 baffles and 7 tubes made of stainless steel. Hot fluid flows inside the tube and cold fluid flows over the tube in the shell. 4 K-type thermocouples were used to read the hot and cold fluids inlet and outlet temperatures. Experiments were carried out for various combinations of hot and cold water flow rates with different hot water inlet temperatures. The flow conditions are limited to the lab size model of the experimental setup. A commercial CFD code was used to study the thermal and hydraulic flow field inside the shell and tubes. CFD methodology is developed to appropriately represent the flow physics and the procedure is validated with the experimental results. Turbulent flow in tube side is observed for all flow conditions, while the shell side has laminar flow except for extreme hot water temperatures. Hence transition k-kl-omega model was used to predict the flow better for transition cases. Realizable k- epsilon model with non-equilibrium wall function was used for turbulent cases. Temperature and velocity profiles are examined in detail and observed that the flow remains almost uniform to the tubes thus limiting heat transfer. Approximately 2/3 rd of the shell side flow does not surround the tubes due to biased flow contributing to reduced overall heat transfer and increased pressure loss. On the basis of these findings an attempt has been made to enhance the heat transfer by inducing turbulence in the shel l side flow. The two baffles were rotated in opposite direction to each other to achieve more circulation in the shell side flow and provide more contact with tube surface. Various positions of the baffles were simulated and studied using CFD analysis and th e results are summarized with respect to heat transfer and pressure loss.

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