Influence of downstream cross-sectional area ratio on flow boiling characteristics of expanding micro pin fin heat sinks

In the present study, influence of downstream cross-sectional area ratio on flow boiling behavior of expanding micro-pin-fin heat sinks was experimentally studied. Three structured micro-pin-fin heat sinks, namely, Type1, Type2 and Type3 were used. Pure saturated conditions were taken into consideration in heat input interval of 90 W - 180 W. To show relation between mass flux and area ratio, two mass fluxes were considered (136 kg m- 2 s- 1 and 250 kg m- 2 s- 1). Outcomes showed that there was an optimum cross-sectional ratio maximizing thermal performance for expanding micro-pin-fin heat sinks. The advantage of prevention of bubble blockage was lost after a threshold expansion ratio due to dominant-compressible-vapor-based instabilities at the downstream region; and in this regard, mass flux played critical role due to liquid inertia force. For the given conditions, compared to uniformly distributed micro-pin-fin heat sink (Type1), expanding heat sinks (Type2 and Type3) led to increase in heat transfer coefficients up to, respectively, 498.9% and 216.5% at 250 kg m- 2 s- 1. Compared to Type1; Type2 and Type3 respectively decreased pressure drop up to 31.8% and 32.1% at 136 kg m- 2 s- 1. However, after heat input of 150 W, Type 3 experienced boiling crisis, and surface temperature jumped.