Analysis of thermal energy transmission path of electric heating elements for intelligent household appliances

- Aug 19, 2020-

Analysis of thermal energy transmission path of electric heating elements for intelligent household appliances:

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In order to calculate the energy efficiency of electric heating elements, it is necessary to analyze the three ways of heat transmission: conduction, convection and radiation. In the actual heat transfer process, these three ways are often accompanied. Different types of electrical appliances, the main effective way of transmission is different. 

3.1 conduction in microcosmic aspect, heat conduction includes three ways: 1) the result of collision when gas gas molecules move irregularly; 2) liquid and nonmetal crystal rely on lattice thermal vibration wave transmission; 3) metal solid depends on electron migration. 

The heat conduction of electric heating element is realized by the third way. The traditional electric rice cooker is a common electric appliance which is dominated by the conduction way of heat energy. The heating source of the traditional electric rice cooker is embedded in the metal chassis. according to Clausius' second law of thermodynamics, ultimately transfers heat to the cooler food by heating the pan body. In this mode, the effective utilization of heat energy is closely related to the structure and material of the whole machine. Good contact reduces additional heat dissipation. But an ideal flat contact does not exist. According to the heat contraction conduction principle, the greater the contact pressure is, the greater the actual thermal contact conduction between the two contact surfaces is, the smaller the thermal resistance is, the greater the thermal conductivity between the two surfaces is, and the enhanced conductivity is. Furthermore, the field coordination principle also plays a role: under certain conditions of velocity field and temperature gradient distribution, the included Angle (field coordination Angle) between the two has an important influence on convective heat transfer intensity. The smaller the included Angle is, the higher the heat transfer intensity will be.

Therefore, in the electrical conduction pathway dominated, compared with the higher electric heat conversion rate, rather than only from the electric heat pipe to improve the electric heat conversion rate, it is better to choose a better thermal conductive material and design a better thermal structure on the whole machine, more can get a good energy efficiency grade.

3.2 Convection

Convection occurs in a liquid or gas in which the temperature tends to be uniform by circulating between the hotter and cooler parts of the liquid or gas. Convection is a special way of heat transfer between liquid and gas. The convection of gas is more obvious than that of liquid. Convection can be divided into natural convection and forced convection. Natural convection often occurs naturally, and is caused by uneven temperature. At present, when adding hot water in domestic electric water heater, it is basically natural convection. Forced convection is caused by external agitation of the fluid, such as the blast of a blower in a hair dryer. Increasing the flow speed of liquid or gas can accelerate the convective heat transfer. The more intense the fluid turbulence, the more it can reduce the thermal resistance of boundary layer and enhance the conduction.
Convection Heater

3.3 Heating Radiant:

In the actual measurement process, different types of heating elements can measure theconversion efficiency of electrothermal radiation by different methods. Based on these test results, we can modify the emissivity and other parameters of the element by changing the surface roughness, metal oxidation degree, element temperature and other processes, so that the element is suitable for different applications.
3.3.1 Distribution photometry
For heating elements that meet or approximately meet the Conditions of the Lambert radiator, the radiometer detector can be used for distribution photometric measurement.thesurface elements are uniformly selected from the measured radiation surface, and thecenter of the surface element is taken as the measuring point of the heating element. When electrification reaches heat balance, the measured values of radiation power Pi and electric power Pe at each measured point are measured and recorded in turn.

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3.4 Analysis of heat transfer

Although the thermal conductivity of metal is better than that of gas and liquid, the contact between different solid parts of electrical components is far less than that of gas or liquid to solid, and the thermal resistance between interstitial Spaces is large, so the overall heat transmission efficiency of general conduction is lower than that of convection.

Electric heating elements whose heating medium is gas can be measured by a balanced ambient room calorimeter or a differential test room. The design of a suitable fin helps to obtain a low thermal resistance, and the fin efficiency of some traditional structures can be found in the engineering manual. In the real production process and under complex working conditions, different product structures can obtain different energy efficiency structures.

As for the energy efficiency assessment of resistance thermal elements whose heating medium dominated by conduction and convection is liquid, it can be simplified to the assessment of heat transfer through convective with liquid, because this assessment method has the most sufficient heat transmission and simple test method. And of course this is based on the fact that the liquid has a better capacity to retain heat without losing heat.

In addition to the infrared heating element, the utilization ratio of infrared radiation in the heat conduction of the heating element is generally not high, so the emissivity of the element is usually reduced to reduce the radiation emittance, so that the heat energy is transferred as far as possible through conduction and convection.

To sum up, the heating elements on the market have different processes and different heating media, and since the actual energy efficiency of the heating elements is greatly affected by the way they are installed, it is not possible to obtain accurate results in a single measurement when we obtain a new sample. With experience, less work can be done, but ultimately the heat transfer has to be analyzed through mathematical calculations of different ways. and can give great help on the improvement of the electric heating elements.