# 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 reported drops of temperature through the thickness of the HMA layer as a function of time [20; 22; 23] are based on constant values of \(\alpha\) and \(\alpha_{\rm z}\). The calculations assume the transfer of heat from the bottom surface to a dry substrate and from the top surface to the air and the compacting roller components (steel or rubber as appropriate). The temperature of air, wind speed and the number of passes and working speed of the compacting roller are taken into account. However, the effect of water is ignored. Under these theoretical assumptions heat should be released primarily through the top surface of the layer (through convection). Apart from conduction, radiation and convection in field conditions there are other phenomena involved in the heat transfer process, which increase the absorption of heat from HMA. They are caused by the presence of water and the physical transformations in which it takes part. The removal of heat related to the presence of water can concern both the bottom and top surfaces of the placed HMA layer and its intensity varies over time. On the underside of the layer water has only a transient effect, even at a high moisture content of the substrate. This is related to a very high amount of heat removed from the interfacial zone (on the bottom surface) that is in direct contact with water. The removed heat is used to heat up the small quantity of water and turn it to vapor. Water vapor moves up making its way through the voids of the not yet compacted HMA. On the way up the water vapor removes heat from the upper strata thus intensifying the cooling process. The amounts of heat removed at the layer underside are at this point much smaller due to the drop of the HMA temperature (to below 100 \({}^{\circ}\)C). Boiling and vaporization do not take place and heat is transferred through conduction and free convection (this allowed by big voids). In addition, increasing the voids content water can affect the bond between the layers. Water coming from precipitation or wetting of steel wheels causes instantaneous drops of temperature in the upper part of the interfacial zone of the HMA layer. This is caused by the removal of heat that is used for heating up water and initiating the boiling and vaporization process. The intensity of the boiling and vaporization processes depends on the difference of the temperature of the hot surface \(T_{S}\) (surface of the HMA layer) and saturation temperature \(T_{n}\) (which depends on the pressure). With moderate heating intensity (heat flux density) heat is transferred by free convection even when the temperature of the heat exchange surface is slightly higher than the saturation temperature. (\(T_{S}-T_{n}=0\)-3 K, Figure 6). Convection currents cause the fluid to move up and vapor is formed only on the free surface of the fluid in the process of surface evaporation. The free convection heat transfer coefficient for fluids is proportional to \((T_{S}-T_{n})^{0.25}\) and the flux density of the absorbed heat is proportional to \((T_{S}-T_{n})^{1.25}\)[10; 15; 17; 18]. An increase of the heat flux density (for greater temperature differences on the heat exchange surface: \(T_{S}-T_{n}=3\)-30 \(K\)) increases the amount of steam bubbles which grow in diameter to come apart and move towards the surface of the fluid (nucleate boiling). This results in vigorous mixing of the fluid, increasing the heat exchange intensity. In the nucleate boiling process the heat transfer coefficient is roughly proportional to \((T_{S}-T_{n})^{2}\) and the absorbed heat flux density is roughly proportional to \((T_{S}-T_{n})^{3}\)[10; 15; 18]. With the temperature difference in excess of 30 K more bubble formation centers develop, which tend to combine, forming larger, irregular bubbles separating the heating surface from the fluid, thus impeding the heat exchange process.