# The Influence of Water on the Heat Loss of Hot Mix Asphalt
## 3 Heat Transfer to the Environment by Combined Convection and Conduction
### Heat Transfer to Water
The changes of the heat transfer coefficient and heat flux density are presented in Figure 6.  
Regardless of the intensity of the heat transfer process the physical changes in water require vast amounts of energy. Boiling is the first-order phase transition and, in constant pressure conditions, a prior absorption of vaporization heat by water is necessary for it to take place. Over 85% of this heat is used in the phase transition process (this due to a very high vaporization heat of water which is 2.26 MJ/kg) and ca. 15% is needed to bring water from the initial to the boiling temperature (Figure 7). Only a small portion of this energy (ca. 0.1%) is used to increase the water vapor temperature. The primary factor determining the energy intensity of the process is the weight of water involved in it. Minimized and controlled use of water can have a decisive bearing on the properties of the HMA layer. The influence of the water temperature on the amount of the energy required for the vaporization is insignificant. As a result of water impact, the surface cools down quickly (within a few dozen seconds) to the temperature below 80-100 \({}^{\circ}\)C in 3-6 mm deep zone (depending on the mixture type). When the temperature has dropped near the surface of the HMA layer any new water will form a thin film atop of it. The amount of water acting on the surface will define the further course of the heat transfer processes. When there is only a small amount of water involved, the process starts with equalization of temperatures between water (in the form of thin film) and the HMA surface and then heat is transferred through conduction (in the water film) and free (natural) convection to air. Further on, the gradual evaporation of water will occur in addition to the above-described processes at a rate depending on the degree of water vapor saturation of the ambient air and the surface temperature of the HMA layer. If a large amount of water is involved (heavy rain) heat will be transferred between HMA and water by free or forced convection--depending on the intensity of the heat transfer process and the flow velocity of water. In the case of porous mixtures water can partly penetrate deeper into the HMA layer continuing intensive cooling of the layer material. The observations during compaction of HMA layers did not reveal any formation of larger bubbles of water vapor on the hot surface or occurrence of pool boiling. The HMA temperature, surface texture and the amount of water are the decisive factors in this respect. Thus we can assume that the difference between the hot surface temperature and the saturation temperature does not exceed 30 K and boiling  
Figure 6: Relationships between the heat flux density q and the heat transfer coefficient \(\alpha\) and the difference between the hot surface/saturation temperatures (\(T_{\rm S}-T_{\rm n}\)) [19; 24; 25]. Legend: K\({}_{1}\)—critical heat flux, K\({}_{2}\)—Leidenfrost point. does not occur. This is primarily due to quick cooling down of the HMA surface (heat source) bringing the heat exchange temperature range to \(T_{S}-T_{n}=0\)-3 K.  
Assuming the constant thickness of water pool the value of natural convection heat transfer coefficient can be determined with equation (6) for laminar flow (\(\propto_{free}^{l}\)) and equation (7) for turbulent flow (\(\propto_{free}^{l}\)) [12]:  
\[\propto_{free}^{l}=A\left(\frac{\Delta T}{d}\right)^{\frac{1}{4}} \tag{6}\]  
\[\propto_{free}^{l}=\frac{0.14}{0.59}\cdot A\cdot(\Delta T)^{\frac{1}{4}} \tag{7}\]  
where: \(A\)--constant defining the type of water flow (Table 1). The values of the A constant determined for the water temperature is the average of the initial water temperature and the initial temperature of the HMA layer.