Roasted chestnuts, roasted peanuts, or roasted sunflower seeds are almost everyone's favorite snacks. However, do you know why so many pebbles or sand are put in the wok when roasting chestnuts, peanuts, or sunflower seeds?
The process of roasting chestnuts is actually a complex one involving various forms of heat transfer such as radiation, convection, and conduction. Firstly, the high-temperature flames transfer heat to the wok; then, through the processes of heat conduction between the wok and the pebbles, between the pebbles and the chestnut shells, and between the shells and the chestnut kernels, the chestnut kernels are gradually heated and roasted. In reality, due to the significant heat loss at each stage, the amount of heat absorbed by the chestnut kernels accounts for only a small fraction of the total heat released during combustion.
Since the specific heat capacity of iron (the amount of heat absorbed to raise the temperature by a certain amount) is relatively low, the iron wok itself absorbs a small portion of the heat and transfers it quickly; while the chestnut shells are just the opposite, with a high specific heat capacity and low thermal conductivity. Therefore, most of the heat is absorbed by the chestnut shells, and only a small portion is transferred to the inner side of the shells to heat the chestnut kernels. This explains the problems with directly using iron woks to roast chestnuts.
Under the heat radiation of the flames, the temperature of the wok reaches hundreds of degrees Celsius. Since the chestnut shells are tightly adhered to the wok and have extremely low thermal conductivity, there is a large temperature difference between the inside and outside of the shells, and the chestnut kernels won't be cooked thoroughly in a short time. However, the chestnut shells are very thin and heat up quickly. As a result, the chestnut shells often char or even catch fire, while the chestnut kernels remain uncooked.
The fundamental reason for this phenomenon is the imbalance and mismatch between the heat conduction processes between the inner and outer walls of the wok and between the inner and outer walls of the chestnut shells.
Therefore, people cleverly add a large number of pebbles. Pebbles have high mass and a thermal conductivity between that of the iron wok and the chestnut shells, which allows them to absorb a large amount of heat without their temperature rising too quickly. In this way, the chestnut kernels have enough time to be roasted at an appropriate temperature.
During the roasting process, it's necessary to constantly stir the pebbles to ensure that the chestnut shells are heated evenly in all directions. In addition, stirring the pebbles can enable them to have more contact with the chestnut shells, which helps to improve the efficiency of heat transfer.