Dumplings always "explode" in the pan? Maybe you need to brush up on your heat transfer skills.

No matter where you are,

Dumplings are always on festive or holiday tables.

Just as indispensable.

If the cooked dumplings are full and complete,

It's chewy and pleasing.

But once it's boiled,

Like a broken work of art,

It's so sad.

Why do dumplings break when cooked?

Is the little trick of adding cold water scientific?

To resolve these doubts,

Let's look at this from a heat transfer perspective.

Cook dumplings scientifically.

1

Basic methods of heat transfer

When talking about heat transfer, we must first introduce the three basic ways of heat transfer: heat conduction, heat convection and heat radiation.

Heat conduction

Heat conduction mainly refers to the heat transfer within or between solids. It refers to the process in which heat is transferred through microscopic particles such as molecules, atoms, and electrons without relative motion between the parts of the material. For example, in the process of cooking dumplings, the heat conduction in the pot wall, the heat conduction in the dumpling skin, and the heat transfer between the dumpling skin and the filling when they are in contact are all heat conduction.

For heat conduction problems, Fourier's law (also known as the basic law of heat conduction) is generally used to describe it, that is, the heat flux density satisfies

Where Φ is the heat flux, which represents the amount of heat flowing through a unit of time, and is expressed in W. A represents the area, so it is natural to see that the heat flux density q represents the amount of heat flowing through a unit surface area per unit of time. dt and dx are the temperature difference and distance microelement, respectively. λ is the thermal conductivity, which is determined by the properties of the material, and is expressed in W/(m·K).

From this expression, we can see three main pieces of information: (1) Heat is transferred from high temperature areas to low temperature areas; (2) The more drastic the temperature change, the faster the heat transfer; (3) The speed of heat transfer is affected by the properties of the material (different thermal conductivity λ).

In the problem of cooking dumplings, we can simply compare the thermal conductivity of the pot wall (iron/stainless steel), water, dumpling skins and dumpling fillings, where the skins and fillings are replaced with wheat flour with a water content of 12% and part of the meat respectively.

Thermal conductivity of different objects | Data: References [2]-[4]

By comparison, it is not difficult to find that the pot, as a metal product, has a thermal conductivity coefficient two orders of magnitude larger than other objects. This is why metal spoons must be equipped with non-metallic handles to avoid burns caused by excessive heat conduction.

Thermal convection

Convection occurs when there is a fluid (liquid or gas) and refers to the heat transfer caused by the mixing of hot and cold parts when the parts of the fluid are in relative motion. Since the internal molecules are also in thermal motion, convection is also accompanied by heat conduction.

Although convection occurs inside a liquid, the process of heat transfer between surfaces when a liquid flows over them is more common in practical applications. To distinguish it from the latter, it is called convective heat transfer. In the process of cooking dumplings, the heat transfer between water and the pot, and between water and the dumpling skin can all be modeled using convective heat transfer.

The convective heat transfer model of the fluid on the surface of the object mentioned above can be described by Newton's cooling formula, written as

Where q is still the heat flux density, and Δt represents the temperature difference between the surface of the object and the fluid. The proportionality coefficient h is called the surface heat transfer coefficient (also known as the convection heat transfer coefficient), and its unit is W/(m2·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 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 according to the cause of flow: natural convection and forced convection.

Natural convection is the flow driven by density differences caused by temperature differences in different parts. For example, when cooking dumplings and the water is not boiling, the water near the bottom of the pot is heated and its density decreases, so it flows to the surface, while the water in the upper layer is lower in temperature and higher in density, and tends to move to the bottom of the pot, forming natural convection.

Forced convection is the flow that occurs when there is a water pump or other pressure difference. For example, stirring the water in a pot with a spoon will cause the water to flow.

Thermal radiation

Thermal radiation refers to the phenomenon that various objects spontaneously radiate electromagnetic waves and release energy due to heat. When electromagnetic waves radiate to other objects, they will be absorbed to a certain extent. Radiation and absorption together constitute radiation heat transfer. When the system reaches dynamic equilibrium, each object absorbs and radiates the same amount of heat.

When discussing radiation, an ideal research object is an object called an absolute black body, which is characterized by being able to absorb all electromagnetic waves that hit its surface. This object has the greatest absorption ability, and obviously, when it reaches dynamic equilibrium, its ability to radiate energy is also the greatest. The energy radiated per unit time by a black body is revealed by the Stefan-Boltzmann law:

Where A is the radiation area, T is the temperature of the black body. σ is the black body radiation constant, which is 5.67×10-8W/(m2·K4). The radiation of an actual object differs from that of an absolute black body by a proportional coefficient ε, that is,

This proportionality coefficient is called the emissivity (also known as blackness) of the object. It is worth noting that the energy of thermal radiation is significantly affected by temperature, so when there is no high-temperature object, thermal radiation is generally ignored.

In the process of cooking dumplings, the flame is a relatively high temperature gas that is completely or partially burning. For example, the temperature of a natural gas stove can generally reach about 1000-1500K, so it can produce obvious thermal radiation. However, as a fluid, the flame itself can also transfer heat to the bottom of the pot by thermal convection, and in general, the latter plays the main role in heating.

After reading this, do you think that the dumplings on the plate actually radiate the light of physics? After all, when cooking dumplings, the heat of the flame goes through the convection heat transfer and radiation heat transfer between the flame and the pot, the heat conduction inside the pot wall, the convection heat transfer between the pot wall and the water, the heat convection and heat conduction inside the water, the convection heat transfer between the water and the dumpling skin, and the heat conduction between the dumpling skin and the dumpling filling before finally cooking the dumplings on our plate. (If you don't think it's physical, you can read the above sentence without taking a breath.) However, we still overlooked an important link, that is, the air in the dumplings.

2

Don't take air as "air"

Reader asks: "Is it important to have air in dumplings?"

The editor replied: "The answer is below."

Readers continue to read the push notifications, and the editor continues to eat dumplings.

——"Chuunisho Gaiden: Heat Transfer"

Let me digress a little. The air inside the dumplings is one of the important reasons why they float when cooked. Since some air is inevitably trapped inside the dumplings, when the dumplings are heated, the air inside will also expand due to the heat. This will cause the total volume of the dumplings at the bottom of the pot to increase, and the buoyancy will increase until the buoyancy can overcome its own gravity and float to the surface.

In fact, the existence of air is related to two things: one is why you need to add cold water to cook the filling, and the other is why dumplings sometimes explode when boiled (Eh!? Didn’t you learn how to fry dumplings by air because you didn’t know how to transfer heat well?).

First of all, the key is that the heat transfer always reaches the dumpling skin first, and then transfers to the dumpling filling inside, so the skin is often cooked quickly, but the filling is not necessarily. As mentioned earlier, as the dumplings are heated, they will be propped up by the hotter air inside, which causes the skin and filling, which were originally in direct contact, to be separated by an air layer.

According to the analysis similar to the previous one, with the addition of the air layer, the heat transfer changes from direct heat conduction between the skin and the filling to convection heat exchange with the air layer and then to the filling. Intuitively, adding a link should reduce the efficiency of heat transfer, but how much does it affect? ​​Let's abstract an ideal model and calculate it.

As shown in the figure, there are two situations with and without an air layer. The heat transfer is simplified into a one-dimensional model. It is assumed that the thickness of the skin and the filling are constant, which are d1 and d2 respectively. The thermal conductivity coefficients of the two are λ1 and λ2 respectively. The temperatures on both sides are constant, which are T0 and T respectively.

In case 1, the interface temperature is Tm. In case 2, the air layer is very thin and the internal temperature is constant, T2, between the inner surface temperature of the skin and the surface temperature of the filling, T1 and T3. The surface heat transfer coefficients of air to the skin and filling are represented by h12 and h23 respectively. Assume that the two systems reach a steady state, that is, the heat flow Φ of each cross section (area A) is the same. According to the previous formula, case 1 has

Situation 2

Adding the temperatures in the two sets of equations gives the relationship

Therefore, we can get

Among them, the denominator

It is called the total thermal resistance of a system, and the reciprocal k1 or k2 is called the total heat transfer coefficient of the system. It can be seen intuitively that when there is an air layer, for the case of a constant temperature difference, the thermal resistance is greater and the heat flow is reduced, which will affect the heating of the dumpling filling.

When cold water is added to boiling water, the water temperature drops suddenly, the gas inside the dumplings shrinks when it is cold, and the dumpling skin and the filling are reattached, which makes the filling more efficiently heated. In addition, if the heating is kept on without adding cold water, the heating efficiency is low. The filling is heated for a long time while the air inside the dumplings is also continuously heated, and there is a possibility that the dumplings will burst due to excessive expansion (explosion.jpg).

In order to more intuitively feel the impact of the air layer, we take d1= 2mm, d2= 1cm, λ1= 0.13W/(m·K), λ2= 0.45W/(m·K) (pure lean meat filling, are you tempted?), in addition, because the surface heat transfer coefficient of air under natural convection is around h=1~10W/(m2·K), we take h12=h23= 5W/(m2·K).

The thin air layer actually reduces the efficiency of heating the filling by an order of magnitude! It seems that if you don’t learn heat transfer well, the dumplings will really explode!!!

Didn't expect that?

There are so many things to learn when cooking dumplings~

Understand the principle behind it.

Now that I have learned so much knowledge,

Why not eat a few more dumplings to reward yourself~

References:

[1] Yang Shiming, Tao Wenquan. Heat Transfer (4th Edition) [Heat Transfer] [M]. Higher Education Press, 2006.

[2] Thermal conductivity of common materials (gkzhan.com)

[3] Thermal conductivity_Baidu Encyclopedia (baidu.com)

[4]Božiková 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)

Editor: Clouds open leaves fall