Heat transfer augmentation in unsteady conjugate thermal systems – Part II: Applications

Abstract

Many energy conversion and other thermal-fluid systems exhibit unsteady convective heat exchange. In such systems, generic spatiotemporal variations in the flow give rise to variations in the heat flux for a given fluid-solid temperature difference, which can be interpreted as spatiotemporal fluctuations of the instantaneous heat transfer coefficient. These variations can lead to unsteady conjugate heat transfer, in which the exchanged heat flux arises from an interaction between the bulk fluid temperature and the temperature in the solid. Further, the nonlinear coupling between the fluctuating temperature differences and the heat transfer coefficient can lead to an effect we refer to as augmentation, which quantitatively describes the ability of a particular arrangement to have a different time-mean heat flux from the product between the mean heat transfer coefficient and the mean temperature difference across the fluid. It is important to be able to understand and to model in a simple framework the effects of the material properties, the geometry and the character of the heat transfer coefficient on the thermal response of the fluid-solid system, and consequently to predict the overall heat transfer performance of these systems. This paper, which follows on from its predecessor (Mathie and Markides, 2012a), is concerned with the phenomenon of augmentation in simple, one-dimensional, unsteady and conjugate fluid-solid systems. A simple semi-analytical one-dimensional model of heat transfer with a time-varying heat transfer coefficient, which was presented in Mathie and Markides (2012a), is applied herein to two different paradigm problems. Such a model can be an important tool in the design of improved heat exchangers and thermal insulation, through for example, the novel selection of materials to exploit these augmentation effects. The first flow considered is a thin, wavy fluid film flowing over a heated plate. This film flow exhibits a periodic fluctuation in the heat transfer coefficient, that is linked to the wavy interfacial deformations of free surface of the liquid film. The second flow considered concerns the heat transfer behind a backwards-facing step, which exhibits broadband fluctuations in the heat transfer coefficient due to the flow separation and turbulence behind * Corresponding author.

Solitary waves on falling liquid films in the inertia-dominated regime

“…Markides and co-workers (Zadrazil, Matar & Markides 2014a; used planar laser-induced fluorescence imaging (PLIF) instead, in order to investigate the coupling between the film-thickness and velocity-field variations over a 2D domain. In Mathie, Nakamura & Markides (2013) and Markides, Mathie & Charogiannis (2015), the same technique was employed alongside infrared thermography (IR), thus allowing for interface-temperature measurements to be retrieved simultaneously with film-thickness measurements. Based on this approach, the authors reported significant heat transport enhancements depending on film topology.…”

“…A schematic of the experimental arrangement, itself an evolution of a previously developed set-up (Mathie et al 2013;Markides et al 2015), is depicted in figure 2. The apparatus consists of a main test section over which film flows develop, and a closed loop via which the liquid circulates.…”

A review of solid–fluid selection options for optical-based measurements in single-phase liquid, two-phase liquid–liquid and multiphase solid–liquid flows

“…The density and surface tension of glycerol solutions can also be modified via the addition of certain alcohols, e.g., ethanol, which has a significantly lower surface tension and density (σ = 23.4 × 10 −3 N/m; ρ = 789 kg/m 3 ), but similar viscosity and RI (µ = 1.2 × 10 −3 Pa s; n = 1.361) compared to water (Vazquez et al 1995;Khattab et al 2012;Mathie et al 2013;Markides et al 2016). Nevertheless, ethanol is known to attack some plastics (e.g., PMMA), so chemical compatibly should be checked at the experiment design stage.…”

Self-similarity of solitary waves on inertia-dominated falling liquid films

“…This is further facilitated by the substantial reduction of the complexity of the governing equations offered by the long-wave nature of the instability. Also, due to their typically small flow rates and low pressure drops, their large contact area and their excellent heat and mass transport characteristics [15,16], falling films are utilized in a wide spectrum of engineering and technological applications, such as evaporators, heat exchangers or chemical reactor columns to name but a few.…”