Numerical Analysis of Water Forced Convection in Channels with Differently Shaped Transverse Ribs

Heat transfer enhancement technology has the aim of developing more efficient systems as demanded in many applications. An available passive method is represented by the employ of rough surfaces. Transversal turbulators enhance the heat transfer rate by reducing the thermal resistance near surfaces, because of the improved local turbulence; on the other hand, higher losses are expected. In this paper, a numerical investigation is carried out on turbulent water forced convection in a ribbed channel. Its external walls are heated by a constant heat flux. Several arrangements of ribs in terms of height, width, and shape are analyzed. The aim is to find the optimal configuration in terms of high heat transfer coefficients and low losses. The maximum average Nusselt numbers are evaluated for dimensionless pitches of 6, 8, and 10 according to the shape while the maximum friction factors are in the range of pitches from 8 to 10

Staggered oriented airfoil shaped pin-fin heat sink: Investigating the efficacy of novel water based ferric oxide-silica hybrid nanofluid

© 2022 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)Nowadays, electronic components are one of the essential parts of almost every smart device. To efficiently transfer the desired amount of heat, recent studies have focused on investigating the potential of advanced thermal coolants and heat sink configurations. Current study reveals the potential of novel water-based hybrid nanofluid of silica (SiO2) and ferric oxide (Fe2O3) for cooling high-heat-generating electronic devices. The experimental work was conducted to inspect the heat transfer characteristics of a uniquely designed staggered oriented airfoil shaped pin-fin heat sink employing Fe2O3-SiO2 hybrid nanofluid with different mixture ratios (25%:75%), (50%:50%), (75%:25%). Airfoil shaped pin-fins offer less resistance to fluid flow and maximum effective area due to their unique shape and delayed separation of fluid at the rear end. All the mixture ratios were tested at three different heating powers (75, 100, 125 W) with varying Reynolds number in laminar flow regime. Experimental results revealed that the fluid having a mixture ratio of 50:50 showed the least thermal resistance followed by 25:75 and 75:25. Maximum enhancement of 17.65% in average Nusselt number was observed against the heating power of 75W. Pumping power was found to increase with the supplementation of nanoparticles in base fluid, while a little variation was observed among different mixture ratios. Finally, the results were compared with recently published studies, which revealed that the airfoil shaped fins have better thermal characteristics and offer less resistance to fluid flow.Peer reviewe

Heat removal in axial flow high pressure gas turbine

The demand for high power in aircraft gas turbine engines as well as industrial gas turbine prime mover promotes increasing the turbine entry temperature, the mass flow rate and the overall pressure ratio. High turbine entry temperature is however the most convenient way to increase the thrust without requiring a large change in the engine size. This research is focused on improving the internal cooling of high pressure turbine blade by investigating a range of solutions that can contribute to the more effective removal of heat when compared with existing configuration. The role played by the shape of the internal blade passages is investigated with numerical methods. In addition, the application of mist air as a means of enhanced heat removal is studied. The research covers three main area of investigation. The first one is concerned with the supply of mist on to the coolant flow as a mean to enhancing heat transfer. The second area of investigation is the manipulation of the secondary flow through cross-section variation as a means to augment heat transfer. Lastly a combination of a number of geometrical features in the passage is investigated. A promising technique to significantly improve heat transfer is to inject liquid droplets into the coolant flow. The droplets which will evaporate after travelling a certain distance, act as a cooling sink which consequently promote added heat removal. Due to the promising results of mist cooling in the literature, this research investigated its effect on a roughened cooling passage with five levels of mist mass percentages. In order to validate the numerical model, two stages were carried out. First, one single-phase flow case was validated against experimental results available in the open literature. Analysing the effect of the rotational force, on both flow physics and heat transfer, on the ribbed channel was the main concern of this investigation. Furthermore, the computational results using mist injection were also validated against the experimental results available in the literature. Injection of mist in the coolant flow helped achieve up to a 300% increase in the average flow temperature of the stream, therefore in extracting significantly more heat from the wall. The Nusselt number increased by 97% for the rotating leading edge at 5% mist injection. In the case of air only, the heat transfers decrease in the second passage, while in the mist case, the heat transfer tends to increase in the second passage. Heat transfer increases quasi linearly with the increase of the mist percentage when there is no rotation. However, in the presence of rotation, the heat transfers increase with an increase in mist content up to 4%, thereafter the heat transfer whilst still rising does so more gradually. The second part of this research studies the effect of non-uniform cross- section on the secondary flow and heat transfer in order to identify a preferential design for the blade cooling internal passage. Four different cross-sections were investigated. All cases start with square cross-section which then change all the way until it reaches the 180 degree turn before it changes back to square cross-section at the outlet. All cases were simulated at four different speeds. At low speeds the rectangle and trapezoidal cross-section achieved high heat transfer. At high speed the pentagonal and rectangular cross-sections achieved high heat transfer. Pressure loss is accounted for while making use of the thermal performance factor parameter which accounts for both heat transfer and pressure loss. The pentagonal cross-section showed high potential in terms of the thermal performance factor with a value over 0.8 and higher by 33% when compared to the rectangular case. In the final section multiple enhancement techniques are combined in the sudden expansion case, such as, ribs, slots and ribbed slot. The maximum heat enhancement is achieved once all previous techniques are used together. Under these circumstances the Nusselt number increased by 60% in the proposed new design

EXPERIMENTAL INVESTIGATION OF INTERNAL COOLING PASSAGES ON GAS TURBINE BLADE WITH PIN-FINS AND RIB-TURBULATORS

Heat transfer and pressure characteristics in a rectangular channel are experimentally explored in detailed. The study consisted of 3 parts: 1) effects of detached pin space, 2) combined effects of detached pin space and ribs, and 3) effects of pin-fin geometry on heat transfer. The overall channel geometry (W=76.2 mm, E=25.4 mm) simulates an internal cooling passage of wide aspect ratio (3:1) in a gas turbine airfoil. With a given pin diameter, D=6.35 mm= ¼E, three different pin-fin height-to-diameter ratios, H/D = 4, 3, and 2, were examined. Each of these three cases corresponds to a specific pin array geometry of detachment spacing (C) between the pin-tip and one of the endwalls, i.e. C/D = 0, 1, 2, respectively. The Reynolds number, based on the hydraulic diameter of the un-obstructed cross-section and the mean bulk velocity, ranges from 10,000 to 25,000. The experiment employs a hybrid technique based on transient liquid crystal imaging to obtain distributions of the local heat transfer coefficient over all of the participating surfaces, including the endwalls and all the pin elements. Pressure drop of each test case is also measured in order to evaluate the performance of each case based on a non-dimensional parameter, performance index, PI. Experimental results reveal that the presence of a detached space between the pin-tip and the endwall have a significant effect on the convective heat transfer and pressure loss in the channel. The presence of pin-to-endwall spacing promotes wall-flow interaction, generates additional separated shear layers, and augments turbulent transport. In general, an increase in detached spacing, or C/D leads to lower heat transfer enhancement and pressure drop. Addition of broken ribs and full ribs has significant impact on heat transfer enhancement at the endwall only. Due to the geometry of the ribs, that is relatively low as compared to the overall height of the channel, the pressure loss seems to be insensitive to the presence of the ribs. Results showed that ribs underperform as compared to the cases without ribs. Triangular pin-fins with sharp edges have the advantages of generating additional wakes and vortices compared to circular and semi-circular pin-fins which contribute to higher heat transfer at the downstream region. However, heat transfer at the leading region of the triangular pin-fins are lower due to a more streamlined geometry at the leading region and without the presence of horseshoe vortices, that is one of the major contributing factors of heat transfer enhancement for circular and semi-circular pin-fins. Having the largest number of pin-fins and arranged in a dense configuration, the TRI3 case has the highest overall heat transfer enhancement ranging between 3.5-3.8, that is approximately 5%-20% higher than that of the circular pin-fin array. As the TRI1 and TRI2 cases show comparable heat transfer enhancement, this suggests that the heat transfer performance of the triangular pin-fin arrays is insensitive to the transverse spacing. In addition, more uniform heat transfer is also observed on the endwall and neighboring pin-fins in all triangular shaped pin-fin arrays. The semi-circular pin-fin array has the lowest heat transfer performance ranging from 2.7-3.4. However, triangular pin-fin arrays give the highest pressure loss due to the largest induced form drag among all cases, while circular pin-fin array exhibits the lowest pressure loss

Computational Study on the Effect of the Staggered Ribs on Heat Transfer Phenomena Between the Horizontal Plates

In terms of multifarious technical applications, various kinds of passive methods are preferred to the active techniques when it comes to increase the amount of convection heat transfer via less energy consumption. As a vortex generator within the scope of the passive method, rib is usually employed to induce the heat transfer enhancement. In this study, the rectangular cross-sectional ribs have been placed to increase the amount of the heat transfer for the staggered arrangement between the horizontal parallel plates. Numerical simulations have been conducted by using k-ω SST turbulence model at Re = 10000. The rib effect has been comparatively investigated in case of thermal and hydraulic performance presented via the numerical results. Time-averaged results for temperature, pressure, streamwise velocity component and streamline patterns have been presented in terms of contour graphics. Furthermore, heat transfer enhancement by using the ribs has been given depending on the increment ratio of Nusselt numbers. Including friction losses due to the ribs mounted on the plates, the values of thermal performance factor for all ducts have been calculated. According to these results for heat transfer augmentation at Re = 10000, h' = 0.1 with S' = 0.5 having η = 1.049 and h' = 0.1 with S' = 0.75 having η = 1.019 have been recommended rather than the smooth duct

Thermal-hydraulic analysis of gas-cooled reactor core flows

In this thesis a numerical study has been undertaken to investigate turbulent flow and heat transfer in a number of flow problems, representing the gas-cooled reactor core flows. The first part of the research consisted of a meticulous assessment of various advanced RANS models of fluid turbulence against experimental and numerical data for buoyancy-modified mixed convection flows, such flows being representative of low-flow-rate flows in the cores of nuclear reactors, both presently-operating Advanced Gas-cooled Reactors (AGRs) and proposed ‘Generation IV’ designs. For this part of the project, an in-house code (‘CONVERT’), a commercial CFD package (‘STAR-CD’) and an industrial code (‘Code_Saturne’) were used to generate results. Wide variations in turbulence model performance were identified. Comparison with the DNS data showed that the Launder-Sharma model best captures the phenomenon of heat transfer impairment that occurs in the ascending flow case; v^2-f formulations also performed well. The k-omega-SST model was found to be in the poorest agreement with the data. Cross-code comparison was also carried out and satisfactory agreement was found between the results.The research described above concerned flow in smooth passages; a second distinct contribution made in this thesis concerned the thermal-hydraulic performance of rib-roughened surfaces, these being representative of the fuel elements employed in the UK fleet of AGRs. All computations in this part of the study were undertaken using STAR-CD. This part of the research took four continuous and four discrete design factors into consideration including the effects of rib profile, rib height-to-channel height ratio, rib width-to-height ratio, rib pitch-to-height ratio, and Reynolds number. For each design factor, the optimum configuration was identified using the ‘efficiency index’. Through comparison with experimental data, the performance of different RANS turbulence models was also assessed. Of the four models, the v^2-f was found to be in the best agreement with the experimental data as, to a somewhat lesser degree were the results of the k-omega-SST model. The k-epsilon and Suga models, however, performed poorly. Structured and unstructured meshes were also compared, where some discrepancies were found, especially in the heat transfer results. The final stage of the study involved a simulation of a simplified 3-dimensional representation of an AGR fuel element using a 30 degree sector configuration. The v^2-f model was employed and comparison was made against the results of a 2D rib-roughened channel in order to assess the validity and relevance of the precursor 2D simulations of rib-roughened channels. It was shown that although a 2D approach is extremely useful and economical for ‘parametric studies’, it does not provide an accurate representation of a 3D fuel element configuration, especially for the velocity and pressure coefficient distributions, where large discrepancies were found between the results of the 2D channel and azimuthal planes of the 3D configuration.EThOS - Electronic Theses Online ServiceEngineering and Physical Sciences Research CouncilGBUnited Kingdo

Investigation of heat transfer augmentation through use of internally finned and roughened tubes : final summary report

This report summarizes a three-year program concerned with obtaining basic design information for tubes having a random roughness on the inside wall (RID) and tubing having continuous internal fins (Forge Fin). Test apparatus and procedures were developed to obtain accurate heat-transfer and friction data for a wide variety of tube geometries using water as the test fluid. For the random roughness the heat-transfer coefficient was above the smooth tube value, for comparable flow conditions, by over 60 percent at a Reynolds number of 30,000. Larger percentage improvements can be expected for higher Reynolds numbers and for fluids having higher Prandtl numbers. Improvements in performance, based on equal pumping power for augmented and smooth tubes, of about 50 percent were observed. The heat-transfer characteristics for tape-generated swirl flow through rough tubes were investigated in order to determine the interaction of swirl flow and roughness effects. For the particular range of parameters covered, for equal flow rates, the maximum improvement in heat transfer with swirl flow in smooth tubes was 70 percent, whereas with swirl flow in rough tubes, the improvement was as much as 100 percent. The heat-transfer coefficient for rough tube swirl flow was accurately correlated by a modification of an additive expression previously suggested for prediction of smooth tube swirl flow data.(cont.) The test program for internally finned tubes established that short spiralled fins produce the greatest improvement in heat transfer. On the basis of equal flow conditions, the heat transfer was improved by over 200 percent; while at equal pumping power, the performance was as high as 170 percent. These improvements, which are attributed to increased area and turbulence promotion, appear to equal the improvements displayed by any of the schemes used to augment heat transfer inside tubes. In order to bring the augmentation problem into perspective, a discussion of data for other types of roughness and finning is included.DS

Numerical simulation of laminar and turbulent hybrid forced-buoyancy convection in channels with a step

The flow over a topography or a step is a fundamental problem in fluid dynamics with relevance to many fields and circumstances. In the present analysis direct numerical simulation (DNS) is initially used to examine the properties of the thermofluiddynamic field in two-dimensional channels with a heated obstruction located on the bottom. The involved dynamics include forced flow driven by injection of cold fluid and the buoyancy convection of thermal origin, which naturally emerges in these channels as a result of the prevailing temperature gradients. The sensitivity of these systems to thermal buoyancy for each considered rate of fluid injection (measured through the related Richardson number, Ri) is explored by varying parametrically the corresponding Rayleigh number (Ra) over a large interval of orders of magnitude (up to the onset of chaos). Different orientations of the step with respect to the forced flow are assumed (Forward-Facing and Backward-Facing Steps) and two alternate paradigms are considered for the bottom of the considered channel, namely an adiabatic or kept-at-constant temperature (hot) boundary. Through this conceptual framework and using a peculiar analysis hierarchy where selected effects are intentionally switched on or off depending on the targeted regime, a kaleidoscope of situations is revealed in the (Ri, Ra) space, which differ in terms of flow patterning behaviour, thermal plume generation phenomena, intensity of heat exchange at the walls and bifurcation scenario. Comparison of forward facing and backward facing step configurations indicates that, besides the differences, these two systems display interesting analogies. These are further explored by removing the constraint of twodimensionality and allowing the flow to develop along the spanwise direction. To reduce the scale of the three-dimensional problem to a level where it is affordable, however, this study is developed in the framework of a large eddy simulation (LES) approach. The results of the three-dimensional simulations are used to clarify some still poorly known aspects, i.e., the dynamics in proximity to the point where the abrupt change in the channel cross-sectional area occurs and the effect of problem dimensionality on the flow behaviour at different length scales.The flow over a topography or a step is a fundamental problem in fluid dynamics with relevance to many fields and circumstances. In the present analysis direct numerical simulation (DNS) is initially used to examine the properties of the thermofluiddynamic field in two-dimensional channels with a heated obstruction located on the bottom. The involved dynamics include forced flow driven by injection of cold fluid and the buoyancy convection of thermal origin, which naturally emerges in these channels as a result of the prevailing temperature gradients. The sensitivity of these systems to thermal buoyancy for each considered rate of fluid injection (measured through the related Richardson number, Ri) is explored by varying parametrically the corresponding Rayleigh number (Ra) over a large interval of orders of magnitude (up to the onset of chaos). Different orientations of the step with respect to the forced flow are assumed (Forward-Facing and Backward-Facing Steps) and two alternate paradigms are considered for the bottom of the considered channel, namely an adiabatic or kept-at-constant temperature (hot) boundary. Through this conceptual framework and using a peculiar analysis hierarchy where selected effects are intentionally switched on or off depending on the targeted regime, a kaleidoscope of situations is revealed in the (Ri, Ra) space, which differ in terms of flow patterning behaviour, thermal plume generation phenomena, intensity of heat exchange at the walls and bifurcation scenario. Comparison of forward facing and backward facing step configurations indicates that, besides the differences, these two systems display interesting analogies. These are further explored by removing the constraint of twodimensionality and allowing the flow to develop along the spanwise direction. To reduce the scale of the three-dimensional problem to a level where it is affordable, however, this study is developed in the framework of a large eddy simulation (LES) approach. The results of the three-dimensional simulations are used to clarify some still poorly known aspects, i.e., the dynamics in proximity to the point where the abrupt change in the channel cross-sectional area occurs and the effect of problem dimensionality on the flow behaviour at different length scales