A Method of Concurrent Thermographic-Photographic Visualization of Flow Boiling in a Minichannel

Abstract

A method is developed to capture the distribution of surface temperature while simultaneously imaging the bubble motions in diabatic flow boiling in a horizontal minichannel. Liquid crystal thermography is used to obtain highly resolved surface temperature measurements on the uniformly heated upper surface of the channel. High-speed images of the flow field are acquired simultaneously and are overlaid with the thermal images. The local surface temperature and heat transfer coefficient can be analyzed with the knowledge of the nucleation site density and location, and bubble motion and size evolution. The horizontal channel is 1.2 mm high × 23 mm wide × 357 mm long, and the working fluids are Novec 649 and R-11. Optical access is through a machined glass plate which forms the bottom of the channel. The top surface is an electrically heated 76 μm-thick Hastelloy foil held in place by a water-cooled aluminum and glass frame. The heat loss resulting from this construction is computed using a conduction model in Fluent. The model is driven by temperature measurements on the foil, glass plate and aluminum frame. This model produces a corrected value for the local surface heat flux and enables the computation of the bulk fluid temperature and heat transfer coefficient along the channel. The streamwise evolution of the heat transfer coefficient for single-phase laminar flow is compared to theoretical values for a uniform-flux boundary condition. Examples of the use of the facility for visualizing subcooled two-phase flows are presented. These examples include measurements of the surface temperature distribution around active nucleation sites and the construction of boiling curves for locations along the test surface. Points on the curve can be associated with specific image sequences so that the role of mechanisms such as nucleation and the sliding of confined bubbles may be discerned.

An application of the non-continuous Trefftz method to the determination of heat transfer coefficient for flow boiling in a minichannel

“…The results obtained with the two methods are very similar. The one-dimensional calculation method proposed by Ozer et al [28] provides results that are similar to those obtained with the one-dimensional method proposed by the authors in [19,22,23].…”

“…9 the curve denoted as "A" (results presented by Ozer at al. [28]) is very similar in shape and values to the curve denoted as "B" (results obtained with the noncontinuous Trefftz method on the basis of the experimental data provided in [28]). Figure 10 compares the local values of the heat transfer coefficient obtained from the authors' experimental data using the proposed two-dimensional calculation Table 3 The differences between the heat transfer coefficient in the saturated nucleate boiling region obtained by means of the non-continuous Trefftz method and the heat transfer coefficient obtained by means of the FEM with the Lagrange interpolation, the Hermite interpolation, the Beck method, for the first 33 % of the measurements Set No.…”

“…One involved applying the two-dimensional calculation model (the non-continuous Trefftz method) to determine the local values of the heat transfer coefficient on the basis of the experimental data provided in [28] concerning flow boiling heat transfer for a coolant liquid in an asymmetrically heated horizontal minichannel and comparing the results with those obtained by Ozer et al [28] (see Fig. 9).…”

“…The results presented in [28] refer to the subcooled boiling region; the local values of the heat transfer coefficient were calculated using the difference between the temperature of the heated surface and the average bulk temperature of the fluid. In Fig.…”

“…(see Fig. 2) [28], the simplified one-dimensional method was based on Newton's Law for convective heat transfer. The local values of the heat transfer coefficient were calculated using the difference between the temperature of the heated surface and the liquid saturation temperature.…”