Jessie Taylor
A candle flame is a type of fire that results from the combustion of a candle's fuel source (wax) and oxygen in the air. This chemical reaction produces light and heat energy, with the latter sustaining the flame by melting the top of the wax, which then moves up through the wick to be continually burnt. The flame's size and burning rate are largely determined by the candle wick, with the type of wax also influencing the burn rate. Candle flames have distinct zones, each with different temperatures and colours, including blue, orange, yellow and white. These colours are caused by varying oxygen levels, with the hottest part of the flame being oxygen-rich and burning blue.
| Characteristics | Values |
|---|---|
| Number of zones | 3 or 5 |
| Zone 1 | Non-luminous zone; insufficient oxygen for fuel to burn |
| Zone 2 | Blue zone; surplus of oxygen, burns clean and <co: 9,11>blue at 800°C |
| Zone 3 | Dark zone; shortage of oxygen, temperature of 1,000°C |
| Zone 4 | Luminous zone; bright yellow, insufficient oxygen for complete burning, temperature of 1,200°C |
| Zone 5 | Veil; oxygen surplus, non-luminous, carbon particles burn faster and more completely, temperature of 1,400°C |
| Shape on Earth | Elongated or teardrop |
| Shape in microgravity | Spherical |
| Hottest part | Blue base |
| Average temperature | 1,000°C (1,800°F) |
| Colour temperature | 1,000 K |
| Light produced | 13 lumens |
| Luminous efficacy | 0.16 lumens per watt |
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What You'll Learn
Candle flame colour zones
A candle flame consists of three or four distinct regions or zones, each with its own colour and temperature. The innermost zone, closest to the wick, is the coolest part of the flame, with a temperature of around 800° Celsius. This zone is black in colour due to the presence of unburnt wax vapours. The middle zone, which is the largest part of the flame, consists of varying shades of yellow and orange. This zone is luminous and emits light due to the incomplete combustion that occurs as a result of limited oxygen supply. The outer zone is blue in colour and is the hottest part of the flame, with a temperature of approximately 1,400° Celsius. This zone has an unlimited supply of oxygen, allowing for complete combustion to take place.
The fourth zone, sometimes called the veil, is the faint blue edge that extends from the base of the flame up the sides of the flame cone. This outer edge is where the flame meets the oxygen in the air directly, contributing to its high temperature. The colour of a candle flame is influenced by factors such as the substance undergoing combustion, the amount of oxygen present, and temperature. The visible yellow colour of the flame is due to radiative emission from hot soot particles, while the blue colour is a result of chemiluminescence.
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Candle flame temperature zones
A candle flame consists of three to four distinct regions, or zones. Each zone has a different temperature and colour, which can help us understand the temperature of each zone.
The first zone is the non-luminous zone, where the fuel on the wick evaporates due to insufficient oxygen, causing an inability to burn. The temperature in this zone is around 600°C.
The second zone is the blue zone, where there is a surplus of oxygen, allowing the flame to burn clean and blue. The temperature in this zone is around 800°C. This zone is responsible for melting the nearby wax, enabling the candle to continue burning.
The third zone is the dark zone, where pyrolysis, or cracking, of the fuel begins due to a shortage of oxygen. This creates minute carbon particles, which give off a yellow light. The temperature in this zone is approximately 1,200°C.
The fourth zone, sometimes called the veil, is the outer edge of the flame. This zone is blue because it directly meets the oxygen in the air and is the hottest part of the flame, reaching temperatures of about 1,400°C.
The colour of each zone is indicative of its temperature, with the hottest part of the flame appearing white or blue, and cooler areas appearing yellow, orange, or red. The average temperature of a candle flame is about 1,000°C.
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Candle flame shape
The flame of a candle is composed of three distinct regions, each with its own temperature and colour, which influence the overall shape of the flame. The innermost part of the flame, near the wick, is oxygen-deficient, resulting in heat-induced reactions. As the wax melts near the wick, it is drawn upwards through capillary action, where it vaporises and combines with oxygen to produce heat, light, water vapour, and carbon dioxide.
The middle part of the flame is the largest and consists of varying shades of yellow and orange. This section emits light, but it is not extremely hot because it receives a limited supply of oxygen, resulting in incomplete combustion. The yellow colour is due to radiative emission from hot soot particles, which are formed through complex chemical reactions involving fuel molecules and molecular growth.
The outer part of the flame is the hottest region, with an unlimited supply of oxygen, allowing for complete combustion. This part of the flame burns with a blue colour and is non-luminous, meaning it does not emit light. The blue colour is a result of chemiluminescence, and the temperature in this zone can reach approximately 1,400°C (2,550°F).
The shape of the candle flame is influenced by the convection current created by the cycle of upward-moving air around the flame. In a quiet environment, a candle flame typically burns steadily in a teardrop shape. However, factors such as wind or a person's breath can interrupt this cycle, causing the flame to flicker or flare, and unburned carbon particles (soot) may escape, resulting in incomplete combustion.
The type of wax used in a candle also affects the shape of the flame. Different types of wax, such as beeswax, coconut wax, paraffin, or soy wax, have varying burn rates, with beeswax and coconut wax burning slower than paraffin or soy wax. Additionally, the size of the flame and the corresponding rate of burning are largely controlled by the candle wick. Modern candle wicks are designed to curve over as they burn, allowing the end of the wick to trim itself through incineration.
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Factors affecting candle flame
A candle flame is a complex phenomenon involving the combustion of wax and oxygen, and it is influenced by various factors. Firstly, the type of wax used in a candle affects the burn rate and flame size. For instance, beeswax and coconut wax tend to burn slower and longer than paraffin or soy wax. The size and type of the wick also play a crucial role in determining the flame's behaviour. If the wick is too large or too long, it can lead to excessive burning, resulting in a larger flame, increased smoke, and reduced candle lifespan. Modern candle wicks are designed to curve over and self-trim through incineration, but older candles required manual trimming with scissors or "snuffers" to maintain a steady flame and prevent smoking.
The environment in which a candle burns is another critical factor. Drafts from open windows, fans, or air conditioning can cause the flame to flicker and move erratically. Placing a candle near flammable items can also pose a fire hazard, especially if the flame height exceeds the recommended range of 0.5 to 2 inches for smaller candles, or in rare cases, up to 3 inches for certain religious candles. The stability of the surface on which the candle rests is essential to ensure even burning.
The chemical composition of the wax and the presence of impurities can also impact the flame's behaviour. All waxes are essentially hydrocarbons, which, when burned, break down into molecules of hydrogen and carbon. However, the presence of impurities in lower-quality wax can cause flickering and affect the burning process. To ensure optimal performance, it is recommended to use high-quality, clean waxes.
Additionally, the colour of a candle flame provides insight into the different temperature zones within it. The hottest part of the flame is the outer region, where complete combustion occurs due to an unlimited supply of oxygen, resulting in a blue colour. The middle part of the flame appears yellow to orange due to incomplete combustion caused by a limited oxygen supply. This section is not extremely hot and emits light. The innermost part of the flame is the smallest and burns with a dull blue colour, reaching temperatures of approximately 1400°C.
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Candle flame in microgravity
Candle flames behave differently in outer space (or microgravity) than they do on Earth. On Earth, gravity-driven buoyant convection causes a candle flame to be teardrop-shaped and carries soot to the flame's tip, which makes it yellow. In microgravity, where convective flows are absent, the flame is spherical, soot-free, and blue. This is because, in microgravity, there is no "up" direction for warm air to rise and create a convection current. Instead, the transport of combustion products and oxygen occurs by the much slower process of molecular diffusion. This diffusion occurs when there is a high concentration of combustion products and a low concentration of oxygen close to the flame, and a high concentration of oxygen farther away from the flame. As a result, a flame in microgravity will often appear to burn less vigorously than a flame on Earth.
In the late 1990s, NASA took candle research to new heights, conducting space shuttle experiments to learn about the behavior of candle flames in microgravity. One study showed that the candle flame lifetimes were around 40 seconds, the flames were dim blue after a transient ignition period, and that just prior to extinction, the flames oscillated spontaneously for about five seconds at a frequency of 1 Hz. The short lifetime of the candle flame was thought to be due to the presence of the small, weakly perforated box that surrounded the candle.
In microgravity, the candle flame is non-propagating and non-convective, with pure diffusion being the only transport mode. It shares characteristics with another classical problem: that of isolated droplet combustion. Given their qualitatively similar flame shapes and the required heat feedback to condensed-phase fuels, the gas-phase flow and temperature fields should be relatively similar for a droplet and a candle in reduced gravity.
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Frequently asked questions
A candle flame consists of three to five zones, each with different temperatures and colours. The innermost zone is oxygen-deficient and is where heat-induced reactions take place. The middle zone is the largest part of the flame and appears yellow or orange due to incomplete combustion. The outer zone is the hottest part of the flame, burning clean and blue due to complete combustion. Some sources further divide the flame into a dark zone where pyrolysis of the fuel begins, and a luminous zone that is bright yellow. A fifth zone, called the veil, is the outer blue edge of the flame.
The hottest part of a candle flame is the outer zone, which has an unlimited supply of oxygen and reaches temperatures of about 1,400 °C (2,550 °F).
The colour of a candle flame depends on the zone being observed. The inner zone is oxygen-deficient and appears dark. The middle zone is yellow or orange. The outer zone is blue.
