Spatiotemporally resolved heat transfer measurements in falling liquid-films by simultaneous application of planar laser-induced fluorescence (PLIF), particle tracking velocimetry (PTV) and infrared (IR) thermography

Abstract

We present an optical technique that combines simultaneous planar laser-induced fluorescence (PLIF), particle tracking velocimetry (PTV) and infrared (IR) thermography for the space-and time-resolved measurement of the film-height, 2-D velocity and 2-D free-surface temperature in liquid films falling over an inclined, resistively-heated glass substrate. Using this information and knowledge of the wall temperature, local and instantaneous heat-transfer coefficients (HTCs) and Nusselt numbers, Nu, are also recovered along the waves of liquid films with Kapitza number, Ka = 180, and Prandtl number, Pr = 77. By employing this technique, falling-film flows are investigated with Reynolds numbers in the range Re = 18 − 66, wave frequencies set to f w = 7, 12 and 17 Hz, and a wall heat flux set toq = 2.5 W cm −2. Complementary data are also collected in equivalent (i.e., for the same mean-flow Re) flows withq = 0 W cm −2. Quality assurance experiments are performed that reveal deviations of up to 2-3% between PLIF/PTV-derived film heights, interfacial/bulk velocities and flow rates, and both analytical predictions and direct measurements of flat films over a range of conditions, while IR-based temperature measurements fall within 1 • C of thermocouple measurements. Highly localized film height, velocity, flow-rate and interface-temperature data are generated along the examined wave topologies by phase/wave locked averaging. The application of a heat flux (q = 2.5 W cm −2) results in a pronounced "thinning" of the investigated films (by 18%, on average), while the mean bulk velocities compensate by increasing by a similar extent to conserve the imposed flow rate. The axial-velocity profiles that are obtained in the heated cases are parabolic but "fuller" compared to equivalent isothermal flows, excluding any wave-regions where the interface slopes are high. As the Re is reduced, the heating applied at the wall penetrates through the film, resulting in a pronounced coupling between the HTC and the film height in thinner film regions. When the imposed wave frequency is increased, a narrower range of HTCs is observed, which we link to the evolution of the film topology and the associated redistribution of the fluid flow upstream of the imaging location, as the liquid viscosity decreases. The local and instantaneous Nu is strongly coupled to the film height and experiences variations that increase as f w is reduced.

Temperature distribution in the cross section of wavy and falling thin liquid films

“…Both the movements of the free surface and its temperature can be monitored by this approach. Furthermore, this technique has also be coupled with velocity field measurement inside the film using PTV technique (Charogiannis et al 2015(Charogiannis et al , 2017Charogiannis and Markides 2019), which gave new insight on the mixing inside of the film with or without wall heating compared to the Nusselt flat film solution, particularly showing that in the presence of wall-heating the flow remains fully laminar. The addition of velocity information is crucial in building a complete picture on what occurs within the films and complements thermal field information such as that obtained in this study.…”

A numerical study of mass transfer from laminar liquid films

“…In order to study the connection between film thickness and mass transfer rate, following Charogiannis & Markides (2019), we show in figure 8 a graph of the dimensionless time-dependent mass transfer rate versus the local film thickness non-dimensionalized by division by for three values of ; time runs counter-clockwise. The dashed line is the flat-film asymptotic value for the corresponding film thickness.…”

“…Several papers report measurements of the local instantaneous heat transfer coefficient and Nusselt number for a falling film flow (see, e.g. Al-Sibai, Leefken & Renz 2002; Markides, Mathie & Charogiannis 2016; Charogiannis & Markides 2019). Although techniques for similar investigations of mass transfer have been developed (see, e.g.…”

“…Furthermore, what has been measured is a heat transfer coefficient based on the rate of the heat flux imposed at the solid boundary and the surface-to-wall temperature difference which, particularly for Prandtl numbers as large as typical Schmidt numbers, can be quite different from the local heat flux at the interface. The majority of the data in the most recent paper (Charogiannis & Markides 2019) refers to situations in which the maximum liquid velocity is smaller than the wave velocity so that recirculation, if at all present, is minimal. The authors observed an increased heat transfer coefficient with respect to steady-film flows although, as they state, ‘the observed heat transfer enhancement does not rely on flow recirculation.’ These results point to a strong effect of the entire flow on the transfer process, which may not be simply reduced to the recirculation/no-recirculation question.…”

Falling film boiling of refrigerants over nanostructured and roughened tubes: Heat transfer, dryout and critical heat flux

“…They have been investigated computationally and experimentally, from fundamental and applied perspectives [1][2][3], in external or internal flow systems with and without heat transfer, in the latter case in an effort to understand the emergence of different wave phenomena. Such waves are known to play a controlling role in affecting flow hydrodynamics and heat transfer [4][5][6] in simple film flows. The case of boiling films is even more complex and requires further attention, with studies of the boiling of stationery thin films [7,8] and thin falling films [9][10][11] having shown the importance of bubble nucleation and microlayer evaporation on the heat transfer process and the accompanying limitations of dryout and critical heat flux.…”