Analytic thermal conductance for square channel, flat plate oscillating heat pipe: CFD simulations of Taylor liquid film and experiment

Abstract

In oscillating (pulsating) heat pipes (OHP, PHP), square channels offer fabrication simplicity (e.g., 3-D printing) and have shown improved thermal performance over circular channels. It has also been suggested that the peripheral averaged slug-deposited liquid film thickness for square and circular cross sections are similar, though the presence of the corners alters the fluid dynamics significantly. Here, using isothermal CFD, the axial development of the Taylor liquid film behind a passing liquid slug in the square capillary channel is predicted. The results show that the film thickness varies peripherally and axially. The heat transfer in the evaporator is simulated by non-isothermal CFD and is dominated by liquid film evaporation. This liquid film conductance is inversely proportional to the peripheral-varying thickness. An analytic, upper-bound OHP thermal conductance is proposed based on an effective liquid film thickness, $G_{{\delta }_l}=\frac{k_l}{{\delta }_{l,sc,c}}{\left(\frac{1}{A_e}+\frac{1}{A_c}\right)}^{-1}$. In a companion experiment, a 3-D-printed square-channel (side dimension of 1 mm) flat-plate OHP (FPOHP) using R-134a as the fluid is tested, and the measured conductance is compared with the predicted upper bound. In FPOHP, the heat conduction between adjacent channels negatively affects the conductance; however, this effect is compensated by enhanced internal conductance. An existing 1-D heat-mass-momentum conserved simulation is extended to the square channel geometry and used to assess this 3-D plate conduction. A reasonable agreement (up to 80 percent) was found between the experiments and the simple, analytic upper-bound OHP thermal conductance.