Chinese New Year's Eve 2022
New Year's Eve
Today is the Chinese New Year's Eve festival,
Presumably, everyone has already posted the Spring League,
Put on a new outfit,
Prepare to go to the homes of relatives and friends for the New Year.
Or sitting around the table,
Have a steaming reunion dinner.
Whether in all corners of the world,
Dumplings are all on the New Year's table,
Essential as well.
If the cooked dumplings are full and complete,
Poop Q bomb is gratifying,
But once it is boiled,
It's like a broken work of art,
It's a shame.
Why do dumplings break?
Is the little trick of adding cool water scientific?
To solve these doubts,
Let's take a heat transfer perspective,
Cook the dumplings scientifically once.
1
The basic way of heat transfer
Speaking of heat transfer, we must first introduce the three basic methods of heat transfer: heat conduction, heat convection and thermal radiation.
Heat conduction
Heat transfer is mainly aimed at heat transfer within or between solids, which refers to the process of heat transfer through microscopic particles such as molecules, atoms, and electrons when the parts of the material do not have relative motion. For example, in the process of cooking dumplings, the conduction of heat in the pot wall, the conduction in the dumpling skin, and the mutual heat transfer when the dumpling skin and the filling are in contact are all heat conduction.
For heat conduction problems, it is generally described by Fourier's law (also known as the basic law of thermal conduction), that is, the heat flow density is satisfied
where the Φ heat flow represents the heat flowing through the unit time, in units of W. A represents the area, so it is natural to see that the heat flow density q represents the heat flowing through the unit surface area per unit time. dt and dx are temperature difference and distance microns, respectively. λ is the thermal conductivity, which is determined by the properties of the material, and the unit is W/(m·K).
From this expression, we can see three main points of information: (1) heat transfer from the high temperature region to the low temperature region; (2) the more intense the temperature change, the faster the heat transfer; (3) the speed of heat transfer is affected by the properties of the material (thermal conductivity λ different).
In the problem of boiling dumplings, we can simply compare the thermal conductivity of the pot wall (iron/stainless steel), water, dumpling skin and dumpling filling, where the skin and filling are replaced by wheat flour with 12% water content and some meat, respectively.
The thermal conductivity of different objects | Data: References[2]-[4]
It is not difficult to find that the pot as a metal product has a thermal conductivity of two orders of magnitude larger than other objects, which is why the metal spoon must be equipped with a non-metallic handle, that is, to avoid burns caused by too fast thermal conductivity.
Thermal convection
Thermal convection occurs in the presence of a fluid (liquid or gas), which refers to the heat transfer caused by the mixing of hot and cold parts when the parts of the fluid are moving relatively between them. Since the inner molecules are also undergoing thermal motion, thermal convection is also accompanied by heat conduction.
Although thermal convection occurs inside a liquid, the process of heat transfer between liquid flowing through the surface of an object and the surface is more common in practical applications, and in order to distinguish the latter is called convective heat transfer. In the process of cooking dumplings, the heat transfer method between water and pot, water and dumpling skin can be applied to the convective heat transfer model.
The convective heat transfer model of the fluid heat transfer on the surface of the object mentioned above can be described using the Newtonian cooling formula, written
where q is still the heat flow density, Δt represents the temperature difference between the surface of the object and the fluid. The scale factor h is called the surface heat transfer coefficient (also known as the convective heat transfer coefficient) in W/(m·K). Since the surface heat transfer coefficient involves the heat transfer between the fluid and the surface of the object, it is not only related to the characteristics of the surface of the object (such as material, shape, size), but also related to the physical properties of the fluid (such as thermal conductivity, heat capacity) and even the flow rate.
Convection can be divided into two types depending on the cause of flow: natural convection and forced convection.
Natural convection is a flow driven by differences in density caused by temperature differences in the various parts. For example, when the dumplings are cooked and the water is not boiling, the density of the water close to the bottom of the pot will decrease after being heated, thus flowing to the water surface, while the water temperature and density in the upper layer are lower, tending to move towards the bottom of the pot, forming a natural convection.
Forced convection is a flow that occurs in the presence of a water pump or other pressure difference, such as stirring the water in the pot with a spoon, which will also cause the flow of water.
thermal radiation
Thermal radiation refers to the phenomenon that various objects spontaneously radiate electromagnetic waves due to heat and thus emit energy. When electromagnetic waves radiate to other objects, they will be absorbed to a certain extent, and radiation and absorption together constitute radiation heat transfer. When the system reaches dynamic equilibrium, each object absorbs and radiates the same amount of heat.
An ideal object of study when discussing radiation is an object called an absolute blackbody, which is characterized by the ability to absorb all electromagnetic waves that hit the surface. The ability of this object to absorb is the greatest, and obviously when the dynamic equilibrium is reached, its ability to radiate energy is also the greatest. The energy radiated by blackbody unit time is revealed by the Styrian-Boltzmann law:
where A is the radiation area and T is the temperature of the blackbody. σ is a blackbody radiation constant with a value of 5.67×10W/(m·K). The difference between the actual object and the radiation of the absolute blackbody is a scale factor ε, ie
This scale factor is called the emissivity of the object (also known as blackness). It is worth noting that the energy of thermal radiation is significantly affected by temperature, so thermal radiation is generally ignored when no high-temperature objects are present.
In the process of cooking dumplings, the flame is a gas that is fully or partially burning with a higher temperature, taking the natural gas stove as an example, its temperature can generally reach about 1000-1500K, so it can produce obvious thermal radiation phenomenon. However, the flame itself, as a fluid, can also transfer heat to the bottom of the pot by means of thermal convection, which generally plays a major role in heating.
Seeing this, do you think that the dumplings on the plate actually emit the light of physics? After all, when cooking dumplings once, the heat of the flame is through convective heat transfer and radiation heat transfer between the flame and the pot, heat conduction inside the pot wall, convection heat transfer between the pot wall and water, heat convection and heat conduction inside the water, convection heat transfer between water and dumpling skin, and heat conduction between dumpling skin and dumpling filling before the dumplings in our plate are finally cooked. (Feeling unphysical, you can read the above sentence without gasping for breath.) But we still overlook the important link, which is the air in the dumplings.
2
Don't really think of air as "air"
The reader asks, "Is it important to have air in dumplings?" ”
The editor replied: "The answer is below." ”
Readers continue to read the push, Xiaobian continues to eat dumplings.
——" "The Transmission of Heat from the Second Institute"
First of all, as an off-topic, the air inside the dumplings is one of the important reasons why the dumplings float when they are cooked. Because the dumplings will inevitably wrap part of the air inside, when the dumplings are constantly heated, the air inside will also be heated and expanded. This will cause the total volume of the dumplings that were originally at the bottom of the pot to become larger, and the buoyancy will become larger, until the buoyancy can overcome its own gravity and then float to the surface.
In fact, the existence of air is also related to two things: one is why it is necessary to add cold water in order to cook the stuffing, and the other is why sometimes dumplings are cooked and fried (eh!?). Didn't you learn to transfer heat to fry? ）。
First look at the first thing, the key is that the transfer of heat always reaches the dumpling skin first, and then to the inside to the dumpling filling, so the skin is often cooked quickly, but the filling is not necessarily. As already mentioned, as heated, the dumplings are propped up by the hotter air inside, which causes the skin and filling that were originally in direct contact to be separated by the air layer.
According to the previous similar analysis, the air layer is added, and the heat transfer is changed from direct heat conduction between the skin and the filling to the filling through convection heat exchange with the air layer. Intuitively, the addition of links should reduce the efficiency of heat transfer, but how much does it affect? Let's abstract an ideal model to calculate.
As shown in the figure, there are two cases where there is no air layer and there is an air layer, which simplifies the heat transfer to a one-dimensional model, and believes that the thickness of the peel and the filling is constant, d and d, respectively, the thermal conductivity of the two is λ and λ, and the temperature on both sides is constant, T and T, respectively.
Case one interface temperature is T. Suppose that the second air layer is very thin, and the internal temperature reaches a constant, T, between the inner surface of the peel and the surface temperature of the filling T and T. The surface heat transfer coefficients of air and pile and filling are expressed in h and h, respectively. It is assumed that the two systems achieve a steady state, i.e. the heat flow Φ of each cross-section (area A) is consistent. By the preceding formula, a scenario exists
Scenario two is
Add the temperatures in the two sets of equations to get the relationship
Hence the availability
Where, the denominator
It is called the total thermal resistance of a system, and the reciprocal k or k is called the total heat transfer coefficient of the system. It can be intuitively seen that when there is an air layer, for the case of a constant temperature difference, due to the greater thermal resistance, the heat flow is reduced, which will affect the heating of the dumpling filling.
When cold water is added to the boiling water, the water temperature can plummet, the gas inside the dumplings shrinks in the cold, and the dumpling skin and the filling are re-pasted, and the heating of the filling will be carried out more efficiently. In addition, if the heating is maintained without adding cold water, the heating efficiency is low, and the air inside the dumplings is constantly heated while the dumpling filling is heated for a long time, and it is possible to burst due to excessive expansion (the gas explodes .jpg).
In order to more intuitively feel the effect of the air layer, we take d = 2mm, d = 1cm, λ = 0.13W/(m·K), λ= 0.45W/(m·K) (pure lean meat filling, hungry?). In addition, since the surface heat transfer coefficient of the air is around h=1~10W/(m·K) under natural convection, h=h=5W/(m·K) is taken. get
The thin layer of air actually reduces the efficiency of the filling being heated by an order of magnitude! It seems that learning is not good at heating, and the dumplings are really going to be fried!!!
I didn't expect it.
There is so much learning about boiling dumplings
Understand the principle behind it,
Dumplings cooked this year's New Year,
Each one should be complete and full!
On this festive occasion,
Once again, I wish all readers a Happy Chinese New Year!
Since you have learned so much knowledge,
You may wish to eat a few more dumplings to reward yourself
Resources:
Yang Shiming, Tao Wenquan. Heat Transfer (4th Edition) [Heat Transfer][M]. Higher Education Press, 2006.
[2] Thermal conductivity of commonly used materials (gkzhan.com)
[3] Thermal conductivity_Baidu Encyclopedia (baidu.com)
[4] Bo?iková M. (2003): Thermophysical parameters of corn and wheat flour. Res. Agr. Eng., 49: 157-160.
[5] Natural gas stove flame temperature_Baidu knows (baidu.com)
Some of the illustrations and elements are from Qianku.com
Edit: Cloud open leaves fall