Why firewood clicks. Generalized repetition of program material in physics in high school in the form of solving illustrated qualitative problems

What associations do you have with the words "burning firewood"? A melted fireplace or stove, a bonfire; pleasant warmth and heat from an open fire; flying sparks and, of course, crackling. Few people think about why firewood cracks. Depending on the type of wood and the degree of its shrinkage, logs can either make almost no acoustic noise, or continuously hiss, click, crack ... In silence, the tree never burns out.

Perhaps, as a child, you loved the books and TV shows in the Fun Physics for Kids series. Simple, everyday occurrences in them were popularly explained empirically. Try to repeat a similar elementary experiment from childhood.

Open a burning stove or sit down by a fire, and then bring a metal ladle, ladle or spoon upside down to the flame. The surface of the metal will soon be covered with droplets. If you bring the dishes even closer to the fire, then it will turn black from soot.

Dip a burning stick into the deep vessel at least twice. The flame will go out, and each time faster. The problem is that carbon dioxide accumulates in the tank. Water, coal and gas - that's what the pile of firewood in your stove will eventually turn into.

It is believed that good firewood (which gives a lot of heat) should be dried before use until the moisture content of the wood is approximately 15-16% (maximum allowed is 25%). However, even the driest tree retains moisture: this is the so-called "capillary" water in cell cavities, and "colloidal" in cell membranes, and a very small amount of chemically bound water.

Maybe during the fires you heard strong pops. These were episodic emissions of hot pyrolysis gases - a process during which the thermal decomposition of wood into various chemical elements occurs. Specialists have even learned how to heat wood in a special way without air (or with its limited access) for various production needs.

So, burning logs crackle and, interestingly, each in its own way. If you put dried beech wood in the stove, you can hardly hear the familiar crackle. The aspen will crackle peacefully, while the pine will simply “shoot” in the flame - the accumulated resin will be released from its cavities and fill the room with a pleasant ether.

Burning firewood not only "talks" in its own way, but also has other distinctive features. For example, the same odorous pine should not be used in an open fireplace - sparks will fly. It is better to burn cherry or apple branches there and enjoy the warmth and pleasant aroma. If you use a home fire or a sauna with a wood-burning stove, be sure to consult a specialist before harvesting firewood.

It is well known that when burning logs (including logs of buildings during fires) hiss, crack, click, “shoot” and even explode. There are many reasons for such acoustic phenomena, and it would be strange if the firewood was “silent” during combustion.

Burning logs crackle and, interestingly, each in its own way. If you put dried beech wood in the stove, you can hardly hear the familiar crackle. The aspen will crackle peacefully, while the pine will simply “shoot” in the flame - the accumulated resin will be released from its cavities and fill the room with a pleasant ether.

When the flame flares up and the firewood is covered with the first charred layer, all the water turns into steam. The continuous crackling of logs in the oven is nothing but a series of real micro-explosions. The charcoal fibers are torn, and water vapor breaks free. Charred wood fragments bounce off each other, making loud clicks. All this indicates the beginning of pyrolysis - a process during which the thermal decomposition of wood into various chemical elements occurs.

Acoustic noise during combustion can be divided into continuous and one-time (episodic). Continuous noises are manifested in the form of hissing (during the turbulent outflow of water vapor and gaseous products pyrolysis from the pores of wood) and in the form of a hum (from turbulent flame combustion). Conventionally continuous noises also include the well-known crackles of wood during combustion, caused by brittle cracks of charcoal across the fibers, so that characteristic crackles occur only after the appearance of a charring layer and clearly indicate the onset of combustion (and not just quick drying).

Episodic clicks are caused by rebounds of fragments (flakes) of the charring layer. In this case, any warping leads to brittle cracking of the coal layer. So even burning paper begins to “rustle” only after charring and with mandatory shrinking and bending of the hot carbon residue. And with warping of massive logs, sharp splits are possible not only in the charred layer, but also in wood, including due to the pressure of water vapor and pyrolysis gases in the depths of the wood. Emissions of hot gases are accompanied by pops and explosions during fires.

However, the reasons for the crackling of wood during combustion are not completely clear. So, some believe that sanded birch cracks much less than aspen when burning, because birch has the smallest difference in tangential and radial shrinkage (and therefore does not warp), while aspen has the largest. At the same time, beech with a high difference in shrinkage does not crack at all during combustion. Therefore, others believe that the harder the wood (and the higher the tensile strength), the less it will crack. In any case, low-strength fir cracks very strongly during combustion. It is difficult to understand and find the true connection between crackling during combustion and cracking. So, the rocks for artistic carving are divided into strongly cracking (hornbeam, ash, maple, beech) and weakly cracking (spruce, pine, fir, cedar, larch).

Aspen is considered extremely controversial wood among the people. If firewood from ash and birch burns even damp, then freshly cut "aspen does not burn without kerosene." Aspen is rarely harvested for firewood, since (like pine) it is used for construction (in particular log cabins for baths). It is believed that aspen is “non-boiling”, gives “little heat”, burns quickly in a dry state and burns soot out of chimneys. Aspen smokes little, it has long been "plucked a torch" for lighting, and now they make matches.

When burning, aspen crackles (“sparks”, “smallly” cracks), unlike pine, for example, which rarely crackles, but powerfully (“shoots”).

Perhaps, as a child, you loved the books and TV shows from the Fun Physics for Kids series. Simple, everyday occurrences in them were popularly explained empirically. Try to repeat a similar elementary experiment from childhood.

Dip a burning stick into the deep vessel at least twice. The flame will go out, and each time faster. The fact is that carbon dioxide accumulates in the tank. Water, coal and gas - this is what the pile of firewood in your stove will eventually turn into.

Maybe during the fires you heard strong pops. It was episodic emissions of hot gases pyrolysis. Simplified, the combustion process consists of two stages: pyrolysis of wood and combustion of its products. Pyrolysis- decomposition of complex organics at temperatures up to 450 °C. At the same time, about a quarter of the products are immediately released in the form of gases: carbon dioxide, hydrogen, methane and carbon monoxide. About half of the mass of wood during combustion forms a liquid consisting of water and various organics, including acetic acid, alcohols and other compounds.

But in a fire or stove, these liquids are invisible, since they instantly evaporate. The rest of the mass forms charcoal, consisting of 80–90% carbon. It is very porous and smolders due to interaction with oxygen penetrating its pores. If the log burns from the surface, then its inner layers warm up and pyrolysis starts in them. The resulting gases accumulate between the layers of wood and tear them apart with a loud crack as the thickness of the outer layer shrinks due to burning through and the pressure underneath increases due to heating. And firewood constantly crackles due to the fact that the wood dries out under the influence of heat and deforms, causing the destruction of the brittle coal that arose on the logs during pyrolysis.

Specialists have even learned how to heat wood in a special way without air (or with its limited access) for various production needs.

Perhaps, as a child, you loved books and TV shows from the Fun for Kids series. Simple, everyday occurrences in them were popularly explained empirically. Try to repeat a similar elementary experiment from childhood.

So, burning logs and, interestingly, each in its own way. If you put in dried beech firewood, you can hardly hear the familiar crackle. The aspen will crackle peacefully, while the pine will simply “shoot” in the flame - the accumulated resin will be released from its cavities and fill the room with pleasant ether.

Burning firewood not only "talks" in its own way, but also has other distinctive features. For example, the same odorous pine should not be used in a fireplace - sparks will fly. It is better to burn cherry or apple branches there and enjoy the warmth and pleasant aroma. If you use a home fire or a sauna with a wood-burning stove, be sure to consult a specialist before harvesting firewood.