How to stop a candle from tunnelling？

Tunnelling phenomenon means that the wax has melted unevenly causing a sunken dip in the candle.
When a candle is lit, the wick creates a flame which burns and vaporizes the wax, converting it into heat and light. When a candle is lit, the wick creates a flame which burns and vaporizes the wax. Meanwhile, it also melts the wax surrounding, causing a little wax pool.
Tunnelling occurs when the flame isn’t hot enough to create a big enough wax pool to completely cover the candle’s surface. Instead the small pool creates a tunnel through the centre of the candle, as the wick burns down, leaving a ring of solid wax around the edges that doesn’t get used.
The key method to stop a candle from tunnelling is to burn a candle evenly.It’s necessary to make sure it stays lit for long enough that the entire surface has melted which is usually between two to four hours.
Besides, it’s useful to ensure an even burn with a candle topper. However, a topper is different from a candle lid, as it has an opening in the centre for the wick to come into contact with air, ensuring the candle can still burn properly.Toppers work by controlling the distribution of heat on the surface of the candle, so that the surface of the wax can melt evenly.
Finally, if there is no candle topper to hand, using tin foil can also fix a tunnelled candle. Take a piece of square foil, fold it several times into a long strip and wrap it around the top of the unlit candle so the edges rise above the rim. Fold or scrunch these edges over at a right-angle, so they shield the solid parts of wax, but there is still a gap in the centre for the wick to be able to burn properly. Light the candle and leave it for a few hours until the candle’s surface is completely melted and even.