Heat transfer is a fundamental concept in physics and engineering that describes the process by which thermal energy is transferred from an object or medium of higher temperature to an object or medium of lower temperature. Understanding heat transfer is important in many areas, from the design of heating and cooling systems to optimizing the performance of electronic devices and industrial processes.
What is heat transfer?
Heat transfer is the process by which thermal energy (heat) is exchanged between objects or systems of different temperatures. This process always takes place from the place with higher temperature to the place with lower temperature, until thermal equilibrium is reached, that is, the temperature between objects or systems becomes equal.
There are three main forms of heat transfer:
- Thermal conductivity: Conduction is a form of heat transfer that occurs due to direct collisions between molecules in an object or between objects in direct contact with each other. Molecules with higher temperatures (moving faster) will transfer energy to molecules with lower temperatures (moving slower), causing the temperatures to equalize. For example, when you touch a hot object, heat will transfer from the object to your hand through conduction.
- Convection: Convection is a form of heat transfer that occurs due to the movement of liquid or gas streams. When liquids or gases are heated, they become lighter and float to the top, while the colder and heavier liquid or gas sinks, forming convection currents that carry heat energy. For example, when water is heated in a kettle, the water at the bottom of the kettle heats up, expands, becomes lighter and floats to the top, while the colder water on top sinks and continues to be heated, forming convection currents.
- Thermal radiation: It is a form of heat transfer that occurs in the form of electromagnetic waves, without the need for an intermediate medium. Any object with a temperature above absolute zero emits thermal radiation. The higher the temperature of the object, the stronger the thermal radiation. For example, the Sun transmits heat to the Earth through thermal radiation, or an electric heater emits heat to warm a room.
Learn the principles of heat transfer
Principle of heat transfer based on the law of conservation of energy and the second law of thermodynamics. The law of conservation of energy states that energy can neither be created nor destroyed, but only transformed from one form to another. The second law of thermodynamics states that heat energy always flows from a region of higher temperature to a region of lower temperature.
Fourier's law of heat conduction
Fourier's law describes the rate of heat transfer through a conducting material. The law is stated as follows:
- Heat flow density (amount of heat transferred through a unit area in a unit time) is proportional to the temperature gradient (change in temperature over distance) and the thermal conductivity of the material.
Formula: q = -k * ∇T
In there:
- q is the heat flux density (W/m²)
- k is the thermal conductivity of the material (W/mK)
- ∇T is the temperature gradient (K/m)
Newton's Law of Cooling
Newton's law of cooling describes the rate of convective heat transfer between a solid object and a surrounding liquid or gas. The law is stated as follows:
- The rate of convective heat transfer is proportional to the contact surface area, the temperature difference between the object and the environment, and the convective heat transfer coefficient.
Formula: Q = h * A * (Ts – T∞)
In there:
- Q is the heat transfer rate (W)
- h is the convective heat transfer coefficient (W/m².K)
- A is the contact surface area (m²)
- Ts is the surface temperature of the object (K)
- T∞ is the ambient temperature (K)
Heat transfer equipment
Heat transfer devices are devices designed to transfer heat from one medium to another efficiently. Heat transfer Efficiency is the primary goal when designing these devices.
Common types of heat transfer equipment:
- Shell and tube heat exchanger: This is the most common type of heat transfer device, consisting of a bundle of tubes placed inside a casing. One fluid flows inside the tubes, while the other fluid flows outside the tubes. Heat is transferred through the tube walls from the hotter fluid to the colder fluid.
- Plate heat exchanger: This type of device consists of a series of thin metal plates stacked on top of each other, forming channels for the fluid to flow through. The plates are designed to maximize the contact surface area, increasing heat transfer efficiency.
- Radiator: Heat sinks are commonly used to cool electronic devices. They are usually made of a highly conductive metal such as aluminum or copper, with fins to increase the surface area exposed to the air. Heat from the electronic device is conducted to the heat sink and then transferred to the air through convection.
- Boiler: A boiler is a device used to heat water or other liquids to produce steam for use in heating systems, power plants, or industrial processes. Heat from the combustion of fuel is transferred to the water through heat transfer surfaces in the boiler.
- Condenser: A condenser is a device used to convert water vapor or other vapors into a liquid. Condensation occurs when the vapor comes into contact with a cooler surface, causing heat to be transferred from the vapor to the surface and the vapor condenses into a liquid.
What is heat transfer coefficient?
Heat transfer coefficient is a quantity that characterizes the heat transfer ability of a material or a system. The higher the heat transfer coefficient, the better the heat transfer ability. Heat transfer Whether it is effective or not depends largely on this coefficient.
Types of heat transfer coefficient:
- Thermal conductivity (k): A quantity that characterizes the thermal conductivity of a material. Unit: W/mK (Watts per meter Kelvin). Materials with high thermal conductivity (such as metals) conduct heat better than materials with low thermal conductivity (such as wood, plastic).
- Convective heat transfer coefficient (h): A quantity characterizing the ability of heat to be transferred between a solid surface and a moving liquid or gas. Unit: W/m².K (Watts per square meter Kelvin). The convective heat transfer coefficient depends on many factors such as the flow rate, the properties of the liquid or gas, and the shape of the surface.
- Overall heat transfer coefficient (U): A quantity that characterizes the overall heat transfer capacity through a system consisting of multiple layers of material or multiple forms of heat transfer. Unit: W/m².K (Watts per square meter Kelvin). The overall heat transfer coefficient is calculated based on the thermal conductivity, convective heat transfer coefficient, and other system parameters.
Convective heat transfer
Convective heat transfer is one of three forms heat transfer convection currents, which occur due to the movement of liquid or gas currents. When a portion of a liquid or gas is heated, it becomes lighter and rises, while the cooler and heavier portion of the liquid or gas sinks, forming convection currents that carry the thermal energy.
Types of convection heat transfer
- Natural convection: Occurs due to density differences caused by temperature differences. For example, the heating of air near the ground during the day creates natural convection currents, carrying warmer air up and cooler air down.
- Forced convection: Occurs due to the action of external factors such as fans, pumps, or wind. For example, using a fan to cool a computer is an example of forced convection. The fan creates a forced airflow, which speeds up the transfer of heat from the hot parts of the computer to the surrounding air.
Applications of convective heat transfer
- Heating and cooling system: Heating and cooling systems use the principle of convection to distribute heat or cool air throughout buildings.
- Cooking: Boiling water, making soup, baking cakes in an oven are all examples of applications of convection heat transfer in cooking.
- Cooling electronic devices: Cooling fans in computers, televisions, and other electronic devices use forced convection to cool heat-generating components.
- Industrial processes: Many industrial processes use convective heat transfer to heat, cool, or dry products.
- Aircraft: Airplane wings are designed based on the principle of convection heat transfer (lift) to help the plane take off.
Conclude
Heat transfer is an important physical process, playing an essential role in many natural phenomena and engineering applications. Understand the forms of heat transfer, operating principles, and factors affecting heat transfer is the foundation for designing and optimizing heat transfer systems, improving energy efficiency, and developing new technologies. From heating homes, cooling computers, to complex industrial manufacturing processes, heat transfer always present and contributing to our lives.
See also:
