Should smoke rise or sink?

Have you wondered why the smoke from the backflow incense could sink? Smoke should rise, shouldn’t it?
At first, I am puzzled too until I found out the mechanism that makes it defined the physics. You might already know this, but since I received some questions from some customers, I thought it was better to share with everyone here.

The structure of a cone incense

Cone vs stick incense

Physically, cone incenses are different to stick (cylinder) incenses.

Stick incense is evenly distributed so once lit up, its burn time is stable from top to bottom. Depending on the manufacturer, its weight and size may vary. The below example is from our incense, 0.14g per stick.

Cone incense is different, as per cone shape, the weight is 1.97g, the 'fuel" (incense content) is 14 times more than the stick.

So what happened when you lit the cone incense?

Have you experienced the cone incense turned off in midway? If so, please read on.

How to light up a cone incense

In this example only, we will divide the cone incense into four parts per below.

The trick is to ensure part 1 is covered in flame first before putting it out, leaving incense burn without the steady fire.

 Tilt the incense down to increase the burn area, ensure the incense is lit up with an intense flame. Wait for 5 to 10 seconds before putting the fire out. For any reasons, if your cone incense is off, re-light it with care, watch out for your finger. If it is unsafe to light the incense as it is too short, please use a multi-purpose lighter with a long wand. 

The image shows the perfect lid.

Do you see the downflow smoke which only happens halfway through the burning?

If you set up correctly, the incense will burn completely without waste, for example, like the one below.

So how does a backflow incense work? 

In general, smoke from combustion (caused by heat) will rise as hot air is less dense than cool air.

So to make smoke descended, an absence of convection is required. The video will explain the point.

The bottle acts as a cone incense.

The paper roll acts as a passage of smoke of the cone incense.

As you see, from the flame (heat source), the smoke rises, but as the smoke travels into the bottle which has no warm air inside, it sinks. This event happens precisely inside the cone incense.

Apologise for the hand drawing diagram. The backflow smoke can be explained per the above picture.

The principle of Atmospheric Pressure (Air Pressure) which we learned from high school

First, let's have a quick look at how the structure of the cone, 

The 3.5cm cone-shaped incense is mainly solid with a small hollow of 2 cm depth passage per below

The depth of the hollow part is approximately halfway through the incense. It means the backflow effect will only occur when the cone is burnt at least at the number 3 mark.

When lit, the flame will cover part 1 and some of section 2, during this time, the smoke will rise as because the heated air is less dense than the surrounding air. The hot air is lighter than cold air which pushes the smoke upward (as part 1 and 2 of the cone is still solid).

When part 2 burns completely, the hollow (part 3) reveals THIS IS WHEN THE BACKFLOW start happening

The air pressure has continuously been pushing down, as there is a hollow part, which allows the smoke to pass through, causing the smoke to flow backward.

This "downflow" (backflow) smoke will rise if disturbed.

You need to ensure the hole of the cone incense in line with the hole of the incense holder to see the full backflow smoke effect. 

There are many different types of cone incenses in the market which all have the backflow effect. Below is our agarwood backflow incense

PLEASE NOTE: ONLY LIT INCENSE IN AN OPEN SPACE WHICH EXCELLENT VENTILATION. DO NOT LEAVE YOUR INCENSE UNATTENDED AS IT IS A FIRE HAZARD.