Heat and Heat transfer

These are two different activities in a heat exchanger.

Heat is thermal energy that is transferred from a hotter object to a cooler object. The amount of heat transferred is determined by the mass of the material (m), the specific heat capacity of the material (c), and the temperature change (?T). Heat Transfer Rate, on the other hand, is the amount of heat transferred per unit time. It is typically measured in watts (W) or kilowatts (kW) and is influenced by factors such as the temperature difference between the two objects, the thermal conductivity of the materials, and the surface area of contact.

?Heat transfer is an essential phenomenon in various processes, where thermal energy is transferred from one medium to another. One of the critical parameters in understanding heat transfer is the heat transfer rate, which quantifies the rate at which heat is exchanged between the different mediums. In a heat exchanger, which is a common device used for heat transfer between two fluids at different temperatures, several resistances need to be overcome for efficient heat transfer.

On the conduction side of the heat exchanger, the thermal conductivity of the material and the boundary layers on either side of the heat transfer surface play a crucial role in determining the rate of heat exchange. Moving to the convection side, the mode of heat transfer and critical heat flux [CHF] can significantly affect the heat transfer coefficient as a mode of boiling switches to film boiling from nucleate boiling. In film boiling the vapor generation near the heating wall can significantly reduce the heat transfer coefficient.

While the temperature difference (ΔT) acts as the driving force for heat transfer, it can also act as a resistance, especially when ΔT between wall temperature and the surface temperature of the fluid reaches such high that there is vapor generation on the heat transfer wall. The vapor begins to act as heat transfer resistance. Heat transfer coefficient a measure for the resistance in the path of heat transfer. The heat transfer coefficient (h) is calculated as the ratio of heat transferred (Q) to the temperature difference (ΔT),

h = Q/ A x ΔT

where (in SI units):

Heat transfer rate (W)

? is the heat transfer coefficient (W/m2K)

?S is urface area is A where the heat transfer takes place (m2)

ΔT is the difference in temperature between the solid surface and surrounding fluid area (K)

Nucleate boiling

It enables efficient heat transfer by utilizing the latent heat of vaporization as the bubbles carry away heat, providing enhanced cooling compared to other heat transfer modes. During nucleate boiling, small vapor bubbles or "nuclei" form on the surface due to local temperature variations. These bubbles grow and detach from the surface, carrying heat away from it. As the heat flux applied to the surface increases, the number and size of vapor bubbles also increase. As the bubbles expand and reach the liquid's surface, convective or evaporative processes cause them to release the heat they have absorbed into the surrounding air. The entire rate of heat transfer between the heated surface and the liquid is accelerated by the processes of bubble formation, growth, and departure.

Film boiling

The entire liquid interface boils on the heating wall, and a continuous vapor film develops on the surface, indicating film boiling. Film boiling is characterized by a stable vapor film formation on the heated surface, providing poor heat transfer.

In film boiling a layer of vapor bubbles forms on the heated surface close to CHF, preventing additional heat transfer. The local wall temperature rises quickly when a heated surface approaches near CHF conditions because the vapour bubbles' presence decreases the area of contact between the heated

surface and the coolant. Consequently, the wall temperature rises and the heat transfer coefficient falls.

The creation of an insulating vapor film on the heated surface is the cause of this decrease in heat transfer.

Critical heat flux [CHF]

Critical heat flux (CHF) is the heat flux at which boiling ceases to be an effective form of transferring heat from a solid surface to a liquid. The use of boiling as a means of heat removal is limited by a condition called critical heat flux (CHF).

The most serious problem that can occur around CHF is that the temperature of the heated surface may increase dramatically due to significant reduction in heat transfer.

Because the heat transfer mechanism shifts from single-phase convective heat transfer to boiling heat transfer at CHF conditions, the near critical heat flux (CHF) temperature of a heated surface increases significantly due to a significant reduction in heat transfer.

In summary, these are key points

1. The formation of a low-density vapor film in film boiling reduces the heat transfer rate by creating a thermal barrier between the heat transfer surface and the bulk fluid. This reduction in heat transfer is due to the lower thermal conductivity of the vapor compared to the liquid. As a result, the heat transfer capacity of the fluid is reduced, leading to a decrease in overall heat transfer rate.

2. As the heat transfer rate decreases in film boiling, more heat is absorbed by the solid wall, leading to a larger temperature difference between the wall temperature and the fluid surface temperature. This increase in temperature difference exacerbates the thermal resistance at the interface between the wall and the fluid, further hindering heat transfer efficiency.

The elevated wall temperature can also lead to safety concerns such as overheating or thermal damage to the heat transfer surface if not properly managed.

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