Heat Transfer Characteristics during Boiling of Immiscible Liquids Flowing in Narrow Rectangular Heated Channels

Abstract

The use of immiscible liquids for cooling of surfaces with high heat generation density is proposed based on the experimental verification of its superior cooling characteristics in fundamental systems of pool boiling and flow boiling in a tube. For the purpose of practical applications, however, heat transfer characteristics due to flow boiling in narrow rectangular channels with different small gap sizes need to be investigated. The immiscible liquids employed here are FC72 and water, and the gap size is varied as 2, 1, and 0.5 mm between parallel rectangular plates of 30 mm × 175 mm, where one plate is heated. To evaluate the effect of gap size, the heat transfer characteristics are compared at the same inlet velocity. The generation of large flattened bubbles in a narrow gap results in two opposite trends of the heat transfer enhancement due to thin liquid film evaporation and of the deterioration due to the extension of dry patch in the liquid film. The situation is the same as that observed for pure liquids. The latter negative effect is emphasized for extremely small gap sizes if the flow rate ratio of more-volatile liquid to the total is not reduced. The addition of small flow rate of less-volatile liquid can increase the critical heat flux (CHF) of pure more-volatile liquid, while the surface temperature increases at the same time and assume the values between those for more-volatile and less-volatile liquids. By the selection of small flow rate ratio of more-volatile liquid, the surface temperature of pure less-volatile liquid can be decreased without reducing high CHF inherent in the less-volatile liquid employed. The trend of heat transfer characteristics for flow boiling of immiscible mixtures in narrow channels is more sensitive to the composition compared to the flow boiling in a round tube.

Thermal and Flow Simulation of Concentric Annular Heat Pipe with Symmetric or Asymmetric Condenser

“…However, the current simulation was calculated by applying the Navier-Stokes equation as a basic theory; if the Knudsen number is greater than 1/100, the Navier-Stokes equation is not valid and can be used as a vacuum condition. The definition of Knudsen number is defined in Equation ( 10), which indicates the number of collisions that occur when a particle passes through a flow region and is assumed to be a continuum if the number of collisions is large enough [27].…”

“…where Ma is Mach number, Re is Reynolds number, and γ is the ratio of specific heat. In the case of the experimental models, the vacuum state was assumed as the Knudson number which was 0.0296 [27].…”

Characteristics of heat transfer under boiling refrigerant transition by pool boiling of immiscible mixtures (Effects of the height of more-volatile liquid layer)

“…1.7 (Sakashita et al, 2010) CHF (Sakai et al, 2010) ( Gordon et al, 1961, Sump andWestwater, 1971) FC-72 0.5-2.0 % 0.2-1.0 % (Roesle and Kulacki, 2012) 1.0 % FC-72 ( ) (Onishi et al, 2013, Okayama et al, 2016 ( Shinmoto et al, 2017) 2 2…”

Thermal Investigations of Hemispherical Shell Vapor Chamber Heat Sink

“…This approach solves by assuming that each phase is a permeable continuum. The VOF (volume of fluid) method is a model for analyzing two or more immiscible fluids and can be employed to simulate the evaporation and bubble condensation in two-phase flows [59][60][61][62]. The VOF algorithm for the current simulation is well described in Ref [52,60].…”

“…The VOF algorithm for the current simulation is well described in Ref [52,60]. In addition, the expression of the phase change used the Lee model in the evaporation-condensation model [52,[60][61][62][63][64][65]. The surface tension between the liquid and vapor is used in the equation proposed Fadhl et al [66] σ = 0.09805856 − 1.845 * 10 −5 * T − 2.3 * 10 −7 * T 2 (47) For the analysis of the HSVC, energy, viscous (SST k-omega) and multiphase (VOF, implicit, Lee model) were applied, and it proceeded for 100 s with a time step of 0.002 s under a pressure-based transient solver and SIMPLE scheme [52,60].…”