Manoj Kumar Moharana

Axial Wall Conduction in Pulsating Laminar Flow in a Microtube

A two-dimensional numerical analysis is carried out to understand the effect of pulsation on the axial wall conduction in simultaneously developing single phase laminar flow in a microtube with constant heat flux boundary condition imposed on its outer surface and while the cross-sectional solid faces exposed to the surrounding are kept adiabatic. Water is used as the working fluid and enters the microtube at 300K with a slug velocity that varying with time sinusoidally, thus causing pulsating flows in the microtube. The inlet velocity thus consists of a fixed component and a fluctuating component which varies sinusoidally with time. For this simulation conductivity ratio is considered at a wide range (ksf 2.26–703) while the thickness ratio (δsf), amplitude (A), and flow rate (Re) remain constant. To understand effect of pulsation, frequency of oscillation (f) is changed by taking four different Womersley numbers (1.414, 2, 2.45, and 3.163). Based on the numerical simulation, it is concluded that for a particular pulsation frequency (Wo) there exists an optimum value of ksf at which overall Nusselt number (Nu) is maximum. Effect of pulsation frequency on heat transfer is found to be very small. Heat transfer is found to be increasing at lower thermal conductive microtube wall material (or ksf) while it is decreasing at higher ksf compared to steady flow in microtube. Existing studies do indicate that pulsation (i) increases heat transfer (ii) decreases heat transfer, or (iii) no effect. The researchers actually failed to observe the present overall trend as none of the existing studies considered a widely varying thermal conductive wall material.Copyright

PHASE-CHANGE HEAT TRANSFER OF ETHANOL-WATER MIXTURES: TOWARDS DEVELOPMENT OF A DISTRIBUTED HYDROGEN GENERATOR

Hydrogen fuel from renewable bio-ethanol is a potentially strong contender as an energy carrier. Its distributed production by steam reforming of ethanol on microscale platforms is an efficient upcoming method. Such systems require (a) a pre-heater for liquid to vapor conversion of ethanol water mixtures (b) a gas-phase catalytic reactor. We focus on the fundamental experimental heat transfer studies (pool and flow boiling of ethanol-water mixtures) required for the primary pre-heater boiler design. Flow boiling results (in a 256 μm square channel) clearly show the influence of mixture composition. Heat transfer coefficient remains almost constant in the single-phase region and rapidly increases as the two-phase region starts. On further increasing the wall superheat, heat transfer starts to decrease. At higher applied heat flux, the channel is subjected to axial back conduction from the single-phase vapor region to the two-phase liquid-vapor region, thus raising local wall temperatures. Simultaneously, to gain understanding of phase-change mechanisms in binary mixtures and to generate data for the modeling of flow boiling process, pool-boiling of ethanol-water mixtures has also been initiated. After benchmarking the setup against pure fluids, variation of heat transfer coefficient, bubble growth, contact angles, are compared at different operating conditions. Results show strong degradation in heat transfer in mixtures, which increases with operating temperature.Copyright

Flow of Taylor Bubble in Microchannel Having an Obstacle

A novel concept of mixing based on 2-D numerical study is proposed where Taylor bubble flows past an obstacle inside a horizontal microchannel. A square shaped obstacle of size 0.02 × 0.02 mm2 is considered inside a microchannel of diameter 0.2 mm. Water and air enters at the two inlet ends of a T-junction and creates Taylor bubble flow at the junction. The obstacle is placed in the downstream at a sufficient distance from the junction where air and water meet. This ensures stability of the Taylor bubble by the time it touches the obstacle. The position of the obstacle is varied along the perpendicular to the flow direction. First, the obstacle is placed exactly at the centre, thus providing equal space of 0.09 mm each on its either side. When Taylor bubble touches this obstacle, it splits and moves through both sides of the obstacle with perfect symmetric flow. The bubbles again join to form the original bubble as it moves past the obstacle. This is inline with the prior expectation. Next, the obstacle is moved by 0.02 mm away from the centre line towards one side, thus providing gap of 0.11 mm and 0.07 mm respectively on the two sides of the obstacle. Now it is found that when the bubble touches the obstacle it do not split in to two, rather the whole bubble moves through the bigger opening of 0.11 mm and only water flows through the smaller opening of 0.07 mm. Similar phenomena is observed when the bubble is further moved away from the centre line towards one side. The liquid-gas interface is found to be continuously changing its shape due to disturbance created by the presence of an obstacle. This causes turbulence inside the liquid plug between two consecutive bubbles, which is confirmed from velocity vector fields. This raises a hope to enhance heat and mass transfer in microchannels by placing multiple obstacles.© 2014 ASME

Conjugate Heat Transfer in Single-Phase Wavy Microchannel

A three-dimensional numerical study has been carried out to understand the effect of axial wall conduction in a conjugate heat transfer situation in a wavy wall square cross section microchannel engraved on solid substrate whose thickness varying between 1.2–3.6 mm. The bottom of the substrate (1.8 × 30 mm2) is subjected to constant wall heat flux while remaining faces exposed to ambient are assumed to be adiabatic. The vertical parallel walls are considered wavy such that the channel cross section at any axial location will be a square (0.6 × 0.6 mm2) and length of the channel is 30 mm. Wavelength (λ) and amplitude (A) of the wavy channel wall are 12 mm and 0.2 mm respectively. Simulations has been carried out for substrate thickness to channel depth ratio (δsf ∼ 1–5), substrate wall to fluid thermal conductivity ratio (ksf ∼ 0.34–646) and flow rate (Re ∼ 100 to 500). The results show that with increase in flow rate (Re), the hydrodynamic and thermal boundary layers are thinned due to wavy passage and they shifted from the centerline towards the peak which improves the local heat transfer coefficient at the solid-fluid interface. It is also found that after attaining maximum Nuavg at optimum ksf, the slope goes downward with increasing ksf for all set of δsf and flow rate (Re) considered in this study.Copyright

Axial Back Conduction through Channel Walls During Internal Convective Microchannel Flows

Recent developments during the last decade in the field of manufacturing and development of many high-power mini-/micro-devices led to increased interest in microfluidic devices involving heat transfer. Since the pioneering work by Tuckerman and Pease (IEEE Electron Device Lett 2:126–129, 1981) on the use of microchannels for high heat flux removal, certainly a lot of developments has been witnessed through ever-increasing analytical, experimental, and highly sophisticated numerical studies by many researchers across the globe. In spite of this progress, many fundamental understandings of flow and heat transfer phenomena in mini-/microchannel systems are still obscure. One such phenomenon is the flow of heat in the solid wall of microchannel systems by means of conduction normally in a direction opposite to that of internal convective mini-/microchannel flow of fluid, called “axial wall conduction” or “axial back conduction.” Axial back conduction is not a new phenomenon, rather mostly neglected unintentionally because of its convincingly smaller influence on heat transfer in conventional-size channels. As the hydraulic diameter of a channel decreases, the coupling between the substrate and bulk fluid temperatures becomes significant because of the relative size of the fluid to the solid wall. Unlike in conventional-size channels, negligence of axial back conduction along the solid walls of micro heat exchangers frequently leads to erroneous conclusions and inconsistencies in the interpretation of transport data. Thus, it is important to explicitly identify the thermofluidic parameters of interest which lead to a distortion in the boundary conditions and thus the true estimation of species transfer coefficients. In this chapter, we focus our attention on the axial back conduction in the solid substrate/channel wall as against the axial back conduction in the liquid flow domain; thus, a detailed review of the state-of-the-art on axial back conduction in both conventional as well as mini-/microchannel systems is presented.