Kuumapistoraja

Kuumapistoraja is a Finnish term that translates to "heat pipe limit" or "critical heat flux" in English. It refers to the maximum heat flux that a surface can sustain while still allowing for efficient cooling through boiling. When the heat flux exceeds the kuumapistoraja, the cooling mechanism changes dramatically, often leading to a sharp increase in surface temperature and a potential for thermal runaway.

This phenomenon is critical in the design and operation of various thermal management systems, particularly those involving phase change cooling. Examples include the cooling of electronic components, nuclear reactor cores, and heat pipes themselves. Understanding and operating below the kuumapistoraja is essential to prevent overheating and component failure.

The exact value of the kuumapistoraja depends on several factors, including the properties of the fluid used for cooling, the surface characteristics of the heat source, and the flow conditions. These factors influence the boiling regime and the efficiency of vapor bubble detachment and removal. When the heat flux is low, nucleate boiling occurs, which is highly effective at heat removal. As the heat flux increases, the surface becomes increasingly covered by vapor bubbles, hindering liquid contact and reducing cooling efficiency. Eventually, a stable vapor film can form, leading to a significant drop in heat transfer capability, which is the onset of the kuumapistoraja.