# Investigation on an Improved Household Refrigerator for Energy Saving of Residential Buildings
## 5 Results and Discussion
### Performances at Different Heat Pipe Design  
In the above calculations, the heat transfer area of the evaporating sections of heat pipes is fixed, and its specific location in the side and the topside freezer wall is also set to the same. In this section, the heat transfer area of the evaporating sections of heat pipes along with the distances between the condensing sections of heat pipes and the inner walls are changed to further evaluate the influences of the heat pipe design. The behaviors of the improved refrigerator are analyzed for ambient temperature of 25 \({}^{\circ}\)C and temperature gear C.  
The heat transfer area of the evaporating sections of heat pipes (\(A_{\textit{eeu}}\)) is firstly reduced from 0.146 m\({}^{2}\) to 0.094 m\({}^{2}\), while the ratio of length to width remains constant. The variations of the average convective heat transfer coefficients of the upper and the lower surfaces of the evaporating sections of heat pipes (\(\alpha_{\textit{eeu}}\)) and the temperature of the heat pipes (\(T_{\textit{HP}}\)) are shown in Figure 12. As the heat transfer area reduces 9.8%, 19.0%, 27.8%, and 36.0%, \(\alpha_{\textit{eeu}}\) raises gradually from 2.68 W\(\cdot\)m\({}^{-2}\cdot\)K\({}^{-1}\) to 2.83 W\(\cdot\)m\({}^{-2}\cdot\)K\({}^{-1}\), 2.78 W\(\cdot\)m\({}^{-2}\cdot\)K\({}^{-1}\), 2.73 W\(\cdot\)m\({}^{-2}\cdot\)K\({}^{-1}\), and 2.89 W\(\cdot\)m\({}^{-2}\cdot\)K\({}^{-1}\), respectively. Correspondingly, \(T_{\textit{HP}}\) decreases from \(-0.7\)\({}^{\circ}\)C to \(-0.9\)\({}^{\circ}\)C, \(-1.2\)\({}^{\circ}\)C, \(-1.4\)\({}^{\circ}\)C, and \(-1.7\)\({}^{\circ}\)C, respectively. The convective heat transfer capability of the evaporating sections of heat pipes per unit area gradually increases as the temperature gap between the heat pipes and the fresh food compartment enlarges. The heat transfer area of the evaporating sections of heat pipes could considerably affect the internal heat transfer of the improved refrigerator. Figure 11: Cold loss recovery behaviors of the improved refrigerator for different temperature gears. Figure 10: Heat flux and temperature of heat pipes versus temperature gear. The impact of heat transfer area reduction of the evaporating sections of heat pipes on the overall behaviors of the improved refrigerator are further shown in Figure 13. When \(A_{eva}\) reduces from 0.146 m\({}^{2}\) to 0.094 m\({}^{2}\), the heat flux of the heat pipes (\(Q_{HP}\)) drops from 3.4 W to 2.8 W, and the heat flux decreasing ratio is about 80.4%. Correspondingly, the cold loss reduction ratio (\(\eta_{re}\)) decreases from 11.2% to 9.3%, and its decreasing ratio is about 82.8%. Results indicate that reduction on the heat transfer area of heat pipes in the fresh food compartment weakens the heat transfer of the heat pipes and thus causes a performance degradation of the improved refrigerator. However, the simulation also proves that a slight sacrifice of heat transfer area for the convenience of design and manufacture of the improved refrigerator is feasible. Thereafter, given the maximum \(A_{eva}\) of 0.146 m\({}^{2}\), the common ratio of wall thickness inside of heat pipes to the whole wall thickness (\(X_{in}\)) is changed from 10% to 25%, and the temperatures of heat pipes and cold reduction ratios of side and topside freezer walls are shown in Figure 14. The temperature of heat pipes (\(T_{HP}\)) raises from \(-\)2.1 \({}^{\circ}\)C to 1.0 \({}^{\circ}\)C with increasing \(X_{in}\). Correspondingly, the cold loss reduction ratio of the side freezer wall (\(\eta_{s,rev}\)) and that of the topside freezer wall (\(\eta_{t,re}\)) gradually decrease from 20.7% to 18.1% and from 18.6% to 12.7%, respectively. Given the same location of the heat pipe, the cold loss reduction capability of the modified side freezer wall remains higher than that of the topside one, but the gap between \(\eta_{s,re}\) and \(\eta_{t,re}\) significantly enlarges from 2.1% to 5.4%.