Shaligram Tiwari

Bio: Shaligram Tiwari is an academic researcher from Indian Institute of Technology Madras. The author has contributed to research in topics: Reynolds number & Heat transfer. The author has an hindex of 18, co-authored 102 publications receiving 908 citations. Previous affiliations of Shaligram Tiwari include Yokohama National University & Indian Institute of Technology Kanpur.

Heat transfer enhancement in cross-flow heat exchangers using oval tubes and multiple delta winglets

Abstract: A three-dimensional study of laminar flow and heat transfer in a channel with built-in oval tube and delta winglets is carried out through the solution of the complete Navier–Stokes and energy equations using a body-fitted grid and a finite-volume method. The geometrical configuration represents an element of a gas–liquid fin–tube cross-flow heat exchanger. The size of such heat exchangers can be reduced through enhancement of transport coefficients on the air (gas) side, which are usually small compared to the liquid side. In a suggested strategy, oval tubes are used in place of circular tubes, and delta-winglet type vortex generators in various configurations are mounted on the fin-surface. An evaluation of the strategy is attempted in this investigation. The investigation is carried out for different angles of attack of the winglets to the incoming flow for the case of two winglet pairs. The variation of axial location of the winglets is also considered for one pair of winglets mounted in common-flow-down configuration. The structures of the velocity field and the heat transfer characteristics have been presented. The results indicate that vortex generators in conjunction with the oval tube show definite promise for the improvement of fin–tube heat exchangers.

Effect of blockage ratio on wake transition for flow past square cylinder

Abstract: Two-dimensional numerical investigations are carried out to study the behavior of wake transition for flow past square cylinder confined between two parallel walls. It is known from past literature that nature of wall confinement affects the behavior of vortex shedding behind the obstacle. The present work reports dependence of the critical Reynolds number (Recr) for onset of planar vortex shedding on blockage ratio. The effect of wall confinement on characteristic features of the wake, such as wake bubble size and shear layer behavior has been studied. The characteristic relationship between Strouhal number and flow Reynolds number has been presented for different values of blockage ratios. The temporal characteristics of the wake are presented.

Numerical prediction of flow and heat transfer in a channel in the presence of a built-in circular tube with and without an integral wake splitter

Abstract: A numerical investigation was carried out to study the heat transfer behavior of a circular tube in cross-flow configuration with a longitudinal fin attached at the rear of the tube. The investigated configuration is intended to model either an element of a cross-flow heat exchanger or an element of the array of pin fins. The longitudinal finning of a circular tube is assumed to be in a configuration where the fin is attached at the back of the circular tube. The longitudinal fins, built-in with the tubes, are called integral splitter plates. The splitter plate creates a streamlined extension of the circular tube. It brings about enhancement of heat transfer from the tube surface. A reduction in the size of the wake zone in comparison with the wake of a circular tube is observed. Narrowing of the wake zone reduced convective heat transfer from the tube surface but the splitter plate itself generated an extra fin area for conduction. Overall, there is an improvement in heat transfer past the circular tube with an integral splitter plate compared with the case of flow past a circular tube without a splitter plate. Flow and heat transfer results are presented for three different chord lengths of the splitter plate and three different values of the Reynolds numbers (500, 1000 and 1500). The heat transfer enhancement obtained by finning was compared with that obtained by increasing the diameter of the unfinned tubes.

Effect of skewness on flow and heat transfer characteristics of a wavy channel

Abstract: Three-dimensional numerical investigations have been carried out to study effect of skewness in wavy channels on flow and heat transfer characteristics. Computations are performed using ANSYS Fluent 16.1 for different values of wave amplitude, skewness angle, Reynolds number (Re) and fixed values of channel width, wavelength. Streamline plots are presented in various planes for different values of skewness angle to understand the flow field characteristics of skewed wavy channels. Volume-averaged as well as span-averaged variation of secondary flow intensity for different skewness angles have been shown to elucidate the strength of induced secondary flow. Effect of secondary flow on heat transfer in skewed wavy channels has been quantified with the help of field synergy angle. Finally, effect of skewness on thermodynamic as well as overall thermo-hydraulic performance of the channel has been presented. Introduction of skewness in the wavy channel induces stronger secondary flow which makes the flow three-dimensional. The stronger secondary flow influences both thermodynamic as well as overall thermo-hydraulic performance of the channel.

Numerical investigations on heat transfer characteristics of curved rectangular winglet placed in a channel

Abstract: Flow and heat transfer characteristics of curved rectangular winglet vortex generators (RWVGs) are investigated numerically. Three-dimensional numerical computations have been carried out for flow through a channel with curved RWVG mounted on its bottom wall. Effect of curvature of RWVG having concave and convex shapes with respect to the flow facing surface, has been addressed in terms of arc angle which is varied in the range from 15° to 120° for fixed arc length. Effect of curvature in concave and convex RWVG on flow and heat transfer characteristics has been compared with that of plane RWVG. Temperature and flow field characteristics near the plate surface have been presented and discussed with the help of temperature contours and streamline plots. Enhancement in heat transfer and pressure loss are examined by using Nusselt number and friction factor respectively. Maximum enhancement in heat transfer is found to be 22% for concave shape RWVG having arc angle equal to 75° as compared to channel in absence of RWVGs. The mechanism of heat transfer enhancement is explored with the help of secondary flow intensity and field synergy principle. Thermodynamic performance of various RWVGs has been analysed by calculating entropy generation rate caused by heat transfer and friction. Finally, overall thermo-hydraulic performance of various curved RWVGs has been reported and compared with that of plane RWVG.

Heat and fluid flow across a square cylinder in the two-dimensional laminar flow regime

Abstract: The flow structure and heat transfer characteristics of an isolated square cylinder in cross flow are investigated numerically for both steady and unsteady periodic laminar flow in the two-dimensional regime, for Reynolds numbers of 1 to 160 and a Prandtl number of 0.7. The effect of vortex shedding on the isotherm patterns and heat transfer from the cylinder is discussed. Heat transfer correlations between Nusselt number and Reynolds number are presented for uniform heat flux and constant cylinder temperature boundary conditions.

Multi-objective shape optimization of a heat exchanger using parallel genetic algorithms

Abstract: We perform in this paper a multi-objective design optimization concerning the blade shape of a heat exchanger, considering the coupled solution of the flow/heat transfer processes. For this, a genetic algorithm is used. The aim of the procedure is to find the geometry most favorable to simultaneously maximize heat exchange while obtaining a minimum pressure loss. An in-house computer package, called OPAL, performs the optimization process in a fully automatic manner. It calls the pre-processor to generate the computational geometry as well as the mesh, it then performs the numerical simulation of the coupled fluid flow/heat transfer problem using Fluent, calculates the output parameters, and iterates the procedure. The genetic algorithm relies on a relatively large number of simulations, which may result in a considerable computational effort, depending on the configuration. The procedure can thus be performed in parallel on a Linux PC cluster to reduce user waiting time. A nearly optimal speed-up is obtained for the present configuration.

A comparative study on the air-side performance of wavy fin-and-tube heat exchanger with punched delta winglets in staggered and in-line arrangements

Abstract: The air-side heat transfer and fluid flow characteristics of wavy fin-and-tube heat exchanger with delta winglets are investigated numerically. The three-dimensional simulations are performed with renormalization-group (RNG) k − ɛ model to lay the foundation for the design of the high-performance heat exchanger. The wavy fin-and-tube heat exchangers which have three-row round tubes in staggered or in-line arrangements are studied. The numerical results show that each delta winglet generates a downstream main vortex and a corner vortex. For the in-line array, the longitudinal vortices enhance the heat transfer not only on the fin surface in the tube wake region but also on the tube surface downstream of the delta winglet; for the staggered array, longitudinal vortices are disrupted at the first wavy trough downstream from the delta winglet and only develop a short distance along the main-flow direction, and the vortices mainly enhance the heat transfer of the fin surface in the tube wake region. The longitudinal vortices generated by delta winglet cause considerable augmentation of heat transfer performance for wavy fin-and-tube heat exchanger with modest pressure drop penalty. When R e D c = 3000 , compared with the wavy fin, the j and f factors of the wavy fin with delta winglets in staggered and in-line arrays are increased by 13.1%, 7.0% and 15.4%, 10.5%, respectively.

An overview on heat transfer augmentation using vortex generators and nanofluids: Approaches and applications

Abstract: The subject of heat transfer enhancement has significant interest to develop the compact heat exchangers in order to obtain a high efficiency, low cost, light weight, and size as small as possible. Therefore, energy cost and environmental considerations are going on to encourage attempts to invent better performance over the existence designs. Streamwise vortices can be generated using small flow manipulators or protrusions such as wings and winglets configurations. Single-pair, single row, or two dimensional array of vortex generators (VGs) can be punched, mounted, attached or embedded in the boundary layer of flow channel. VGs generate longitudinal and transverse vortices, while longitudinal vortices are more efficient for heat transfer enhancement than transverse vortices. A dramatic augmentation in thermal performance of the thermal system can be achieved but pressure drop penalty is existed. Several parameters have been overviewed in this paper, which have pronounced effect on the convective heat transfer coefficient and pressure drop penalty. These parameters are: attack angle of VG, geometry of VG, standard and novel types of VG, spacing between the VG tips, number of pairs of VGs in the flow direction, rectangular or circular array arrangement of VGs, common-flow upper (CFU) or common-flow down (CFD) configuration of VG, pointing up (PU) or pointing down (PD) arrangement of VG with flow direction, Re number, channel aspect ratio, number of tubes of fin-tube heat exchanges (HE), circular or oval tubes of fin-tube HE, and location of VG respect to the tube of HE or from leading edge of the channel. This paper gives an overview about the early studies done in order to improve the performance of thermal systems with minimal pressure losses to derive systems with less negative impact on the environment and high level of energy economic. This study also provides an outlook for future work using nanofluids with vortex generators. This article is also summarizes the recent experimental and numerical developments on the thermal conductivity measurements of nanofluids, thermal conductivity enhancement, convection and conduction heat transfer, some applications, main problems and suggestions for future works.

Energy-efficient and -economic technologies for air conditioning with vapor compression refrigeration: A comprehensive review

Abstract: Vapor Compression Refrigeration Systems (VCRS) are widely used to provide cooling or freezing for domestic/office buildings, supermarkets, data centres, etc., which expend 15% of globally electricity and contribute to ∼10% of greenhouse gas emissions globally. It is reported that cooling demand is expected to grow tenfold by 2050. Therefore, it is critical to improve the efficiency of the VCRS. In this paper, a comprehensive review of advanced and hot technologies is conducted for the VCRS. These technologies include radiative cooling, cold energy storage, defrosting and frost-free, temperature and humidity independent control (THIC), ground source heat pump (GSHP), refrigerant subcooling, and condensing heat recovery. Radiative cooling could produce a cold source ∼8 °C lower than the surroundings, which reduces the electricity consumption of the VCRS by ∼21%; cold energy storage is used to shift the peak cooling load, and as a result, the electricity consumption and operation cost of the VCRS could be reduced by ∼12% and ∼32%, respectively; frosting is a big issue of the VCRS especially for freezing applications, and more than 60% of electricity consumption for defrosting could be saved with the advanced defrosting and frost-free technologies; THIC deals with the building sensible load and latent load separately, which not only increases the COP of the VCRS by ∼35%, but also improves the building thermal comfort; GSHP uses the ground as a low-temperature cooling source for condensing the refrigerant in the VCRS in summer, which decreases the condensing temperature by ∼5 °C and correspondingly increases the COP of the VCRS by ∼14%; refrigerant subcooling and condensing heat recovery can increase the refrigerating capacity and achieve multi-functions of the VCRS, respectively. The review is summarized in terms of the technology classification, basic ideas, advantages/disadvantages, current research status and efforts to be made in the future.