In the present paper the effect of solid surface thermophysical properties under saturated flow boiling conditions within a considered microchannel is numerically investigated. The simulations are performed by utilising a custom, enhanced VOF-based solver that has been developed in OpenFOAM CFD Toolbox and accounts for conjugate heat transfer between solid and two-phase fluid domains. The properties of stainless steel, copper and silver are considered, utilising a single rectangular microchannel having a hydraulic diameter 𝐷ℎ of 200μm. Due to the high computational cost, a channel length L of only 4.80 mm is used for all cases. The working liquid is ethanol, while the applied heat and mass fluxes are 20 kw/m2 and 150 kg/m2s, respectively. Two different series of simulations are performed. In the first series the focus is on a single nucleation site and a single nucleation event, tracking the flow path and the growth characteristics of a single bubble. In the second series multiple nucleation sites with four subsequent nucleation events are examined. In the first series, the numerical simulation predictions indicate that the alternation of the solid surface properties, do not affect the resulting bubble dynamics and an elongated vapour slug is formed for all cases. Under the examined conditions, a small difference on the local time-averaged heat transfer coefficient between the different cases is observed, with the silver channel showing the highest (3.93%) and the copper channel showing the lowest (2.55%) increase in the global heat transfer coefficient, compared to the corresponding single-phase values that constitute the initial condition for each case. From the second series of simulations, it is evident that multiple nucleation sites with multiple nucleation events lead to more significant enhancement of the heat transfer coefficient with respect to the reference single-phase cases but also in significant heat transfer performance differences, comparing the different channels to each other.
A numerical study on flow boiling within micro-passages: the effect of solid surface thermophysical properties
Marco Marengo
2021-01-01
Abstract
In the present paper the effect of solid surface thermophysical properties under saturated flow boiling conditions within a considered microchannel is numerically investigated. The simulations are performed by utilising a custom, enhanced VOF-based solver that has been developed in OpenFOAM CFD Toolbox and accounts for conjugate heat transfer between solid and two-phase fluid domains. The properties of stainless steel, copper and silver are considered, utilising a single rectangular microchannel having a hydraulic diameter 𝐷ℎ of 200μm. Due to the high computational cost, a channel length L of only 4.80 mm is used for all cases. The working liquid is ethanol, while the applied heat and mass fluxes are 20 kw/m2 and 150 kg/m2s, respectively. Two different series of simulations are performed. In the first series the focus is on a single nucleation site and a single nucleation event, tracking the flow path and the growth characteristics of a single bubble. In the second series multiple nucleation sites with four subsequent nucleation events are examined. In the first series, the numerical simulation predictions indicate that the alternation of the solid surface properties, do not affect the resulting bubble dynamics and an elongated vapour slug is formed for all cases. Under the examined conditions, a small difference on the local time-averaged heat transfer coefficient between the different cases is observed, with the silver channel showing the highest (3.93%) and the copper channel showing the lowest (2.55%) increase in the global heat transfer coefficient, compared to the corresponding single-phase values that constitute the initial condition for each case. From the second series of simulations, it is evident that multiple nucleation sites with multiple nucleation events lead to more significant enhancement of the heat transfer coefficient with respect to the reference single-phase cases but also in significant heat transfer performance differences, comparing the different channels to each other.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
