Zi-Qin Zhu

Heat Transfer During Constrained Melting of Graphite-Based Nanofluids in a Spherical Capsule

Transient heat transfer during constrained melting of graphite-based solid-liquid phase change nanofluids in a spherical capsule was investigated experimentally. Nanofluids filled with self-prepared graphite nanosheets (GNSs) were prepared at various loadings up to 1% by weight, using a straight-chain saturated fatty alcohol, i.e., 1-dodecanol (C12H26O), with a nominal melting point of 22 °C as the base fluid. In-house measured thermal properties were adopted for data reduction, including thermal conductivity, dynamic viscosity, latent heat of fusion, specific heat capacity and density. A proper experimental approach depended on volume expansion was figured out to monitor the melting process of nano-enhanced phase change fluid in a spherical capsule indirectly and qualitatively characterize the process. A variety of boundary temperatures were also adopted to vary the intensity of natural convection. It was shown that under low boundary temperatures, a monotonous melting acceleration came into being while increasing the loading due to the monotonously increased thermal conductivity of the nanofluids. While increasing the boundary temperature leads to more intensive natural convection that in turn slowed down melting under the influence of nanoparticles because the contribution by natural convection is significantly suppressed by the dramatically grown dynamic viscosity, e.g., more than 60-fold increase at the loading of 1 wt.%. The melting rate is determined by the competition between the enhanced heat conduction and deteriorated natural convection.Copyright

An Experimental Study of Thermal Performance of a Two-Phase Loop Thermosyphon (TPLT)-Based Steam Generator: Effects of Thermal Boundary Conditions

For the Parabolic trough collector (PTC) system, thermal boundary condition of the receiver (or heating section) is important for the thermal optimization. In this work, effects of thermal boundary on thermal performance of the two-phase loop thermosyphon (TPLT) natural circulation PTC system was investigated experimentally. Three kinds of thermal boundary heating conditions (upper and lower half, and whole circular heated) and two filling ratios (FR = 0.6, 1.2) were adopted in this paper. The results show that half heating condition can improve heat transfer performance in receiver and system thermal resistance. But the preferred half heating boundary was varied as the filling ratio was changed. However, a lower thermal efficiency was observed for the partly heating boundary conditions. For a low heat flux condition in this work, the effects of thermal boundary on flow instability were not obvious, especially for the bigger filling ratio condition.Copyright