What Is the Working Principle of a Dewar Flask?

The working principle of a Dewar flask is based on suppressing the three basic modes of heat transfer—conduction, convection, and radiation—through its unique structural design, achieving excellent thermal insulation. Here’s a detailed explanation:

1. Suppressing Heat Conduction

1.1 Double-layer Vacuum Structure

The Dewar flask consists of inner and outer containers made of metal or glass, with a vacuum in between. This vacuum layer contains almost no gas molecules, so heat cannot be transferred through molecular collisions (i.e., conduction), significantly reducing heat flow from the outside to the low-temperature liquid inside.

1.2 Low Thermal Conductivity Support

The inner and outer layers are supported by materials with low thermal conductivity (such as fiberglass, ceramics, or plastic) to avoid direct contact, further minimizing heat conduction.

2. Suppressing Heat Convection

2.1 Vacuum Layer Eliminates Convection

Since there are no gas molecules in the vacuum layer, convection currents cannot form, thus preventing heat transfer by convection.

2.2 Narrow Neck Design

The neck of the Dewar flask is long and narrow, reducing the contact area between the low-temperature liquid and the ambient air, further limiting convective heat exchange.

3. Suppressing Heat Radiation

3.1 Reflective Coating

The inner surface of the Dewar flask is usually coated with a highly reflective metal layer (such as silver or aluminum), which reflects most infrared radiation and reduces radiant heat absorption.

3.2 Multi-Layer Insulation (Optional)

Some advanced Dewar flasks use multi-layer insulation (MLI), where multiple reflective screens are placed in the vacuum layer to further reduce the effects of heat radiation.

4. Summary of the Working Principle

The Dewar flask suppresses heat transfer through:

• Vacuum layer: Eliminates conduction and convection.

• Reflective coating: Reduces radiation.

• Narrow neck: Minimizes conduction and convection.

• Low-conductivity supports: Further reduces conduction.

5. Performance in Practical Use

Due to its excellent insulation, the Dewar flask significantly slows down the evaporation rate of cryogenic liquids. For example, liquid nitrogen boils at -196°C under standard atmospheric pressure. In a Dewar flask, it can be stored for days or even weeks at room temperature with relatively low evaporation loss.

6. Example

Assume liquid nitrogen is poured into both a regular container and a Dewar flask:

• Regular container: The liquid nitrogen boils off rapidly, likely evaporating completely within minutes.

• Dewar flask: The liquid nitrogen remains in liquid form for days or even weeks, with a much slower evaporation rate.

This difference clearly demonstrates the superior heat insulation of the Dewar flask.

7. Conclusion

The Dewar flask works by using a double-walled vacuum structure and reflective coatings to suppress conduction, convection, and radiation, achieving highly efficient thermal insulation. This makes it an ideal tool for storing and transporting cryogenic liquids, widely used in scientific research, medical applications, and industrial fields.