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The magic of heat flow: How is it quietly transferred between air and objects?
In physics and engineering, heat flux, or heat flux density, sometimes called heat flux intensity, is the amount of energy flowing per unit area per unit time. The unit for this concept is watt per square meter (W/m2). Heat flow has direction and magnitude and is therefore considered a vector quantity. Whether in science or engineering, the proper measurement and application of the concept of heat flow is critical to understanding and improving various thermal management systems.
Heat flow is everywhere, both in our daily lives and in high-tech engineering applications.
Definition and Application of Heat Flow
To define the heat flow at a point in space, we usually consider infinitesimal surface states. Heat flow is often represented by
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, and the subtagq indicates that this is heat flow, not mass or momentum flow. Fourier's law is one of the important laws describing heat flow, especially in solid materials, where heat is mainly transferred by conduction.
Fourier's law tells us that heat flow always flows from high temperature areas to low temperature areas.
Fourier's law and its extensions
The expression of Fourier's law in one dimension is:
φq = -k * dT(x)/dx
Where k is the thermal conductivity. The negative sign indicates that the heat flow is toward the area of lower temperature. When we explore multi-dimensional situations, heat flow still follows similar principles:
Here, ∇ is the gradient operator, which shows the relationship between heat flow and temperature gradient.
Methods for measuring heat flow
There are several ways to measure heat flow. First, we can measure the flow of heat through a material of known thermal conductivity, similar to measuring the voltage drop across a known resistor when measuring electrical current. However, this approach is relatively difficult to implement in practical situations because the thermal resistance of the material is usually unclear, so accurate material thickness and thermal conductivity are crucial.
When thermal conductivity is unknown, a heat flow sensor is an effective measurement tool.
Another way to measure heat flow is to use a heat flow sensor or heat flow transducer, which directly measures the amount of heat transferred by the surface it is mounted on. Therefore, even though the values of thermal resistance or thermal conductivity do not need to be known, we can perform in situ measurements.
Applications of thermal flow sensors
Common heat flow sensors such as differential temperature thermopiles have a basic principle similar to the first measurement method mentioned above, but do not require knowledge of the thermal resistance or thermal conductivity of the product. This makes thermal flow sensors more flexible in many practical applications. However, these thermopiles must be calibrated to convert their output signal (usually measured in microvolts) into a heat flow value (measured in watts per square meter per Kelvin). Once calibrated, the heat flow sensor can be used to measure heat flow directly without the need for known thermal resistance or thermal conductivity.
In science and engineering, energy balance is an important tool for analyzing heat flow and thermal management systems.
Thermal Flow Analysis in Science and Engineering
Scientists and engineers often rely on the tool of energy balance, which can be applied to any physical system, from chemical reactors to biological organisms. The energy balance formula is as follows:
∂Ein/∂t - ∂Eout/∂t - ∂Eaccumulated/∂t = 0The three items here represent the total energy change entering the system, the total energy change leaving the system, and the cumulative change in energy within the system. If the only way energy is transferred is by heat flow, then the heat flow rate can be used to calculate the energy balance. Even in practical applications, where it is not possible to know the exact heat flux at every point on the surface, we can use approximate schemes to perform calculations, such as the Monte Carlo integration method.
With the advancement of technology, the importance of heat flow measurement and control has become increasingly prominent. It plays an important role in various applications such as thermal management of buildings, heat dissipation design of electronic equipment, and many industrial processes. In this rapidly changing world, can we further understand the mysteries of heat flow to advance science and technology?
