Shaligram Tiwari

Numerical Prediction of Flow and Heat Transfer in a Rectangular Channel With a Built-In Circular Tube

A numerical investigation of flow and heat transfer in a rectangular duct with a built-in circular tube was carried out for moderate Reynolds numbers and varying blockage ratios. Since the heat transfer in the duct is dictated by the flow structure, the study was directed towards characterization of the flow. To this end, the topological theory shows promise of becoming a powerful tool for the study of flow structures. The limiting streamlines on the tube and the bottom plate reveal a complex flow field. The separation lines and points of singularity (saddle points and nodal points) were investigated. The iso-Nusselt number contours and span-averaged Nusselt number distribution in the flow passage shed light on the heat transfer performance in the duct. The investigation was necessitated by the need to enhance heat transfer in fin-tube heat exchangers through identification of the zones of poor heat transfer. The predicted results compare well with the well documented experimental results available in the literature. In the range of Reynolds numbers considered for the present case, no need is felt to employ any turbulence model in order to describe the heat transfer behavior. Time series signals of the transverse velocity component in the wake zone are presented with their FFT and time-delay plots. The onset of turbulence is not observed up to the highest value of the Reynolds number considered in the present case. This confirms that the transition to turbulence is delayed in the present case compared with that observed for flow past a circular tube placed in an infinite medium. The reason may be attributed to the narrow gap between the no-slip channel walls.

A numerical study of heat transfer in fin-tube heat exchangers using winglet-type vortex generators in common-flow down configuration

Numerical investigation of flow and heat transfer in a rectangular duct with a built-in circular tube in the presence of delta-winglet type vortex generators has been carried out for moderate Reynolds numbers and three different angles of attack of the winglets (24?, 30? and 34?). The investigation is necessitated by the need to enhance heat transfer on the gas side of the fin-tube heat exchangers through generation of streamwise longitudinal vortices. The strategy for heat transfer enhancement involves introduction of a strong swirling motion in the flow field. The swirling motion can be generated by the longitudinal vortices. In the present study, the longitudinal vortices have been created by the delta-winglet type vortex generators in common-flow-down configuration. The vortex generators are mounted behind the tubes. An element of a heat exchanger has been considered for detailed study of the flow structure and heat transfer analysis. The present numerical study is limited to isothermal fins, single row of tubes and laminar flow situations. The analysis can easily be extended to multiple rows of tubes. It has been separately verified that the flow situation retains its laminar nature for the range of Reynolds number considered in the present problem. Hence the heat transfer analysis does not use any turbulence model in the present investigation. The consequences of employing these vortex generators are a significant separation delay and removal of the zone of poor heat transfer from the near-wake of the tubes. The heat transfer in the near-wake region can be enhanced as high as 230%. Results also show a marked increase in overall heat transfer in the channel.

Numerical Investigation of Laminar Unsteady Wakes Behind Two Inline Square Cylinders Confined in a Channel

Abstract Two-dimensional numerical investigations have been carried out to study the behavior of unsteady wake for flow past an inline arrangement of square cylinders confined in a channel. The present work aims to study the influence of the relative size and arrangement of the two cylinders on vortex shedding characteristics in their wakes. A computational code has been developed using finite difference approach based on modified MAC algorithm for solution of governing flow equations. Effect of size ratio of the two cylinders, the cylinder spacing in streamwise direction and flow Reynolds number on the wake flow characteristics of the two cylinders have been presented. The temporal characteristics of the wake are analyzed using time series analysis. Results indicate that for a given Reynolds number, there exist a range of values of size ratio and cylinder spacing for which vortex shedding can occur in the wake of the upstream cylinder.

Numerical study of heat transfer and pressure drop for flow past inline and staggered tube bundles

Purpose – The purpose of this paper is to develop an indigenous three‐dimensional computational code and apply it to compare flow and heat transfer characteristics for inline and staggered arrangement of circular tubes in a tube bundle.Design/methodology/approach – A finite‐volume based computational code is developed to solve the momentum and energy equations for flow through a three‐dimensional rectangular channel and past built‐in tube bundles having inline and staggered arrangement. The approach is based on SIMPLE algorithm. The basic conservation equations of mass, momentum and energy are solved over a body‐fitting grid on the physical domain to obtain the flow and temperature fields.Findings – Heat transfer and pressure drop are compared for inline and staggered tube arrangements in a tube bundle over range of Reynolds numbers 300 ≤ Re ≤ 800. Results are validated suitably against those available in literature.Research implications – Tube‐fin heat exchangers with continuous fins on a tube array are ...

Numerical Study of Flow and Heat Transfer for Flow Past Inline Circular Tubes Built in a Rectangular Channel in the Presence of Vortex Generators

A three-dimensional numerical study has been carried out for laminar flow of air past an inline arrangement of circular tubes built-in with a rectangular channel in the presence of vortex generators. The governing equations of momentum and energy are solved with the help of a developed computational code based on finite-volume technique. Computed flow and temperature field results are analyzed with an aim to find an optimum location of vortex generators resulting in the maximum enhancement of heat transfer causing a minimum increase in pressure drop. To find the optimized location of vortex generators, the effect of variation of the Reynolds number on flow and heat transfer has also been studied.

Effect of arrangement of inline splitter plate on flow past square cylinder

Two-dimensional numerical study has been carried out to investigate the effect of the presence of a single splitter plate in upstream location, and presence of two splitter plates, in upstream as well as downstream locations, for low Reynolds number incompressible flow past a square cylinder. Using finite difference discretisation and MAC algorithm, an indigenously developed computational code has been used for solution of the governing flow equations. The length of the plate and its position do play good role in controlling the flow past the cylinder. The difficulty lies in identifying an optimum configuration of these parameters. The length of the attached splitter plate as well as separation of the detached splitter plate of fixed length are varied for both the upstream and downstream configurations up to maximum Reynolds number of 200. To begin with, the effect of upstream splitter plate alone in attached as well as detached configuration is studied with an aim to identify the optimum configuration associated with reduced drag as well as delayed onset of vortex shedding. Then, for the optimum configuration of the upstream plate, the effect of downstream plate on flow characteristics and critical Reynolds number for onset of vortex shedding has been studied.

Flow past two tandem square cylinders vibrating transversely in phase

Numerical investigations have been carried out to study the wake characteristics of flow past two tandem square cylinders vibrating in phase. Both the cylinders vibrate in a transverse direction, i.e., perpendicular to the incoming flow with the same frequency and amplitude. The frequency of vibration of the cylinders and the inter-cylinder spacing are varied for fixed values of the Reynolds number (Re = 100) and the amplitude ratio ( = 0.4). The synchronous or lock-in regime for the oscillatory wake of the vibrating cylinders has been identified by varying the frequency of the vibration from = 0.4 to 1.6 ( being the frequency of vortex shedding behind a stationary square cylinder). The characteristics of lift and drag and the mechanism of vortex shedding are studied by varying the excitation frequency within the lock-in range for each value of inter-cylinder spacing. The complex interaction of flow between the cylinders gives rise to a variety of characteristically different shedding patterns in their wake. For values of inter-cylinder spacing equal to 2D and 3D, periodic, as well as quasi-periodic, lock-in behaviors are observed in the synchronous range.

Effect of Vortex Generators and Integral Splitter Plate on Heat Transfer and Pressure Drop for Laminar Flow Past Channel-Confined Tube Banks

Three-dimensional numerical computations are carried out to study heat transfer and pressure drop characteristics for laminar flow past circular tube banks in staggered and in-line arrangement confined within a narrow rectangular channel. A finite-volume based computational code has been developed for the computations. Vortex generators mounted near the upstream tube promote fluid mixing and enhance heat transfer in stagnant wake zone behind the tubes but offer extra pressure drop penalty. A splitter plate attached to the rear of the upstream circular tube acts as an extended heated surface and is expected to reduce the extra pressure drop due to vortex generators. The objective of this study is to find the configuration of tube arrangement in the presence of vortex generators and splitter plate that would be associated with enhanced heat transfer and reduced pressure drop. Heat transfer and pressure drop characteristics are presented for different lengths of the splitter plate in the absence and presence of vortex generators for both in-line and staggered arrangement of circular tubes.

Effect of Rectangular Winglet Pair in Common Flow Down Configuration on Heat Transfer from an Isothermally Heated Plate

ABSTRACT Present three-dimensional numerical study aims to investigate the effect of mounting rectangular winglet pair (RWP) on heat transfer enhancement in flow over a flat plate. Computations for incompressible flow of air have been carried out using commercial software ANSYS Fluent. Flow of air has been considered over the surface of an isothermally heated horizontal plate in presence of RWP in the range of Reynolds number from 400 to 2000. Common flow down configuration of RWP has been considered to study the effect of various geometric parameters, such as length of RWP, spacing between leading edges of the winglets and angle of attack of RWP, on flow characteristics and enhancement in heat transfer. Flow and temperature field characteristics have been presented using streamlines and temperature contours near the plate surface and streamlines in cross-stream planes. Enhancement in heat transfer in presence of RWP has been quantified using cross-stream variation of local Nusselt number, streamwise variation of span-averaged Nusselt number and surface-averaged overall Nusselt number.

Performance study of beacon-enabled IEEE 802.15.4 standard in WSNs with clustering

To support high scalability and better data aggregation, sensor nodes are often grouped in to disjoint and mostly non overlapping sub sets called clusters. Clusters create hierarchical WSNs which facilitate efficient utilization of limited resources of sensor nodes and thus extends network lifetime. ZigBee, a communication standard for LR-WPAN (Low Rate Personal Area Networks), has low power consumption, complexity and cost. ZigBee uses the IEEE 802.15.4 standard as its communication protocol for Medium Access Control (MAC) layer and physical (PHY) layer. The IEEE 802.15.4 MAC standard achieves duty cycle Operation by setting two system parameters, macBeaconOrder (BO) and macSuperframeOrder (SO), to achieve low power consumption for different value of duty cycle. In this Paper we have analyzed cross layer (MAC/PHY) approach for wireless sensor network scenario with clustering in which we consider LEACH(Low energy adaptive clustering hierarchy protocol) algorithms as clustering protocol and DYMO (Dynamic MANET on-demand) as a routing protocol and analyze improvement of clustered over not clustered WSNs. We have calculated the attributes like average jitter, end to end delay, throughput and total charge consumption for different % of duty cycle and have found improvement in network latency and average jitter.