## 4 Result and Discussion
Figure 7: Variation of cooling capacity. At the entrance of the evaporator, the tubes are mostly filled with the liquid phase of the refrigerant. As a result of the heat flow through the tube wall, the internal energy of the liquid increases, which leads to an increase in turbulence and the heat transfer coefficient in the inlet area. It is characteristic of the Kandlikar model that the maximum value of the heat transfer coefficient occurs at a vapor quality of \(x\) = 0.5-0.6 for bubbly flow and nucleate boiling. The dominant region of nucleate boiling (bubbly region) is characterized by high heat transfer coefficients and low dependence on steam quality, since the wall temperature is nearly constant. The region where weak nucleate boiling predominates (slug region) is characterized by a decrease in heat transfer coefficients with increasing steam quality. In this region cyclic drying and rewetting of the tube wall occurs at the top and lateral ends of the tube. In the flow boiling region, convective boiling becomes more important, with a smaller contribution from nucleate boiling, where the liquid phase is usually present only in the lower half of the tube. As the wall temperature increases, the vapor density also increases, while the viscosity of the vapor decreases in contrast. These two facts together result in a significant increase in Reynolds number, which leads to increased forced convection heat transfer through the steam at the top wall of the tube. The bottom half of the tube continues to contain the stratified liquid in which nucleate boiling takes place. When the quality of the steam increases, the contribution of nucleate boiling decreases, and the heat transfer by forced convection of the steam is dominant. In this region, the heat transfer coefficient continues to decrease. This is followed by a drying region of high steam quality, characterized by a small decrease in the heat transfer coefficient. In the region of steam superheating, the heat transfer coefficient is low and almost constant, while the temperature of the tube wall increases faster. It can be seen that the refrigerant is superheated in the last 45 cm of the evaporator. In the superheated region, the values of the single-phase heat transfer coefficient are low and almost the same for all temperatures of inlet water. The pressure drop of the refrigerant is continuous along the evaporator (Figure 9). The pressure drop of the refrigerant flowing in the evaporator tubes was \(\Delta p\) = 0.41 bar at a water inlet temperature of 13 \({}^{\circ}\)C, while at a water inlet temperature of 19 \({}^{\circ}\)C it was \(\Delta p\) = 0.297 bar. Figure 8: Variation of the heat transfer coefficient of the refrigerant. Considering the equation of the state of an ideal gas and the p-h diagram of a heat pump, the explanation of Figure 10 is clear. When the temperature of the water at the inlet of the evaporator rises, the temperature of the refrigerant also rises. This increases the pressure in the system. When the pressure increases, the condensing temperature also increases. Consequently, the temperature and vapor content of the refrigerant is also higher after the expansion valve (inlet to the evaporator). The temperature drop of the evaporating refrigerant along the evaporator length is 3 \({}^{\circ}\)C at a water temperature of 13 \({}^{\circ}\)C, while at 19 \({}^{\circ}\)C the temperature drop is only 1.6 \({}^{\circ}\)C. The values for the temperature of the two-phase refrigerant show a decreasing tendency because the viscous friction is present that cause pressure drop. But the temperature of the refrigerant in the vapor phase in the superheating region increases spectacularly (2.76 m to 3 m evaporator length). In the following Figures 11-14, measurement results are compared with the values obtained from the mathematical model presented.