Heat transfer enhancement by layering of two immiscible co-flows A.-R.A. Khaled a , K. Vafai b,⇑ a King Abdulaziz University, Mechanical Engineering Department, Jeddah 21589, Saudi Arabia b University of California at Riverside, Mechanical Engineering Department, Riverside, CA 92521, USA article info Article history: Received 31 August 2013 Received in revised form 17 September 2013 Accepted 17 September 2013 Keywords: Heat transfer Enhancement Immiscible fluids Minichannel Convection abstract Enhancement of heat transfer in minichannels due to co-flowing of two immiscible fluids in a direct con- tact is investigated in this work. Different fluid combinations are analyzed. The momentum and energy equations for both flows are solved analytically and numerically. The numerical and analytical solutions are found to be in good agreement. A parametric study including the influence of fluids relative viscosity, thermal conductivity, thermal capacity and height ratios is conducted for various Peclet numbers. Differ- ent ranges of the parameters that augment the heat transfer are obtained, and different physical aspects of the problem are discussed. For practical fluid combinations with small Peclet numbers, the enhance- ment factor can increase up to 2.6 folds. However, that increase is about 1.2 folds when the Peclet number is increased by two orders of magnitude. This work establishes the mechanisms for heat transfer enhancement utilizing two immiscible co-flows. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction A review of recent heat transfer literature [1–3] reveals that large number of investigations was devoted to the topic of heat transfer enhancement. Most of these works considered at least one of the following enhancement mechanisms: (a) stream profil- ing [4,5] such as using twisted tapes, spirals, and wire coils, (b) fins such as slotted and louvered fins [6,7], (c) electrohydrodynamic effects [8], (d) surface coatings [9], (e) additives [10,11], (f) acoustic streaming [2], (g) turbulators [2] and, (h) flow and velocity ampli- fications [12–14]. Additional reviews in this area have shown an increased interest in enhancement technology [15,16]. A number of these works introduce new concepts in this area [17–22]. Heat transfer Enhancement utilizing an immiscible fluid co- flowing with the coolant flow in a direct contact manner has not received much attention by researchers [1–3,15,16,22]. As such, this subject is considered as the main topic of the present work. Immiscible fluids having smaller viscosity or larger specific heat than the coolant can enhance the heat transfer. This is because they can enhance the velocity near the heated plate, widening the thermal entry region or amplifying the coolant flow rate. These effects are the major causes for heat transfer enhancement by lam- inar flows. Therefore, the present work is additionally aimed to specify ranges for thermophysical properties and flow conditions that can result heat transfer enhancement by co-flowing two immiscible fluids. One of the main advantages of heat transfer enhancement by co-flowing of two immiscible fluids in direct contact manner is that both flows are subject to the same pressure gradient. In contrast, the pressure gradient of the counter-flowing system decrease to zero then changes sign and starts to decrease below zero. As a re- sult, the fluid flow rates in the co-flowing system are expected to be larger than those in the counter-flowing system. Moreover, for short channels with co-flowing system, the secondary fluid tem- peratures at the inlet of the primary flow are much smaller than those of the counter-flowing system. This effect elevates the con- vection heat transfer coefficient between the heated boundary and the adjacent primary flow due to thermal entry region. Finally, as the micro-channel geometry may cause inadequate operation of efficient enhancement methods [23–25], co-flowing of two immis- cible fluids can become an attractive alternative. In the present work, heat transfer through two layered immis- cible fluid flow within a horizontal minichannel is analyzed. For all analyzed cases, the density and the thermal conductivity of the primary fluid residing on the heated boundary are considered to be larger than those of the other fluid, referred to as the second- ary fluid. The dynamic viscosity and the volumetric thermal capac- ity of the secondary fluid are allowed to be either larger or smaller than those of the primary fluid. Both momentum and energy trans- port equations are solved for each flow using analytical and numerical methods. Various analytical solutions for the tempera- ture field under applicable constraints are obtained and validated against the numerical solution. A parametric study of the heat transfer enhancement is made to identify ranges of controlling parameters that reveal favorable enhancement attributes. 0017-9310/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijheatmasstransfer.2013.09.040 ⇑ Corresponding author. Tel.: +1 951 827 2135; fax: +1 951 827 2899. E-mail address: vafai@engr.ucr.edu (K. Vafai). International Journal of Heat and Mass Transfer 68 (2014) 299–309 Contents lists available at ScienceDirect International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt