# Thermal loss definition in CENOS

Heat transfer is a very important part of the simulation physical setup - without correct heat exchange definition accurate results cannot be acquired. There are different parameters to define heat exchange, each with specific values that depend on material properties and cooling methods.

## Heat transfer types and values​

CENOS offers 4 types of heat exchange definitions - convection, radiation, heat flux, and heat flow. Each of these definitions can be used for specific applications, depending on temperature and cooling approach.

### Convection​

Convective heat transfer, often referred to simply as convection, is the transfer of heat from one place to another by the motion of fluid or gas. Convection is usually the dominant form of heat transfer in liquids and gases, and plays a significant role through a wide range of heat treatment applications. Convective heat transfer is defined with $h$ - Heat Transfer Coefficient $( W/m^2K )$ and the $T_0$ - Ambient temperature $(^{\circ} C)$. Coefficient $h$ measures the intensity at which the heat is removed from the surface by the fluid or gas with temperature $T_0$. The appropriate Heat Transfer Coefficient value is very important for correct simulation definition. It ranges from small to very large numbers and can be confusing. Heat Transfer Coefficient depends on the chosen cooling approach (natural or forced convection) and the fluid used in the process. The approximate values of different colling approaches are as follows:

Forced convection :$W/m^2K$ :
Low speed flow of air over a surface10
Moderate speed flow of air over a surface100
Moderate flow of water in a pipe3000
Water and liquids50 to 10'000
Natural convection :$W/m^2K$ :
Gases and dry vapors5 to 37
Gas flow in tubes10 to 350
Water and liquids50 to 3000
Air10 to 100
Water100 to 1200
Water flowing in tubes500 to 1200
Oil50 to 350

It is possible to define Heat Transfer Coefficient as temperature dependent for special cooling liquids, such as PAG solutions.

PAG 12% solution spray quench Heat Transfer Coefficient dependance from temperature

Temperature $(^{\circ} C)$ :Heat Transfer Coeff. $(W/m^2K)$ :
332975
593828
997972
14910004
19811264
24511832
29613011
34814799
39817155
44717887
49918415
54418049
59817115
64616099
69913823
74912076
79810045
8487566
8985088

Thermal radiation is electromagnetic radiation emitted from the material. All matter with a temperature greater than absolute zero emits thermal radiation. Radiation is defined with Emissivity and Ambient Temperature. It occurs through vacuum or any transparent medium (solid, fluid, or gas) and plays a big role at temperatures above 800 $^{\circ} C$.

Emissivity coefficient for some common materials can be found in the table below. Emissivity coefficient for some materials varies with temperature - emissivity coefficients given below are at the temperature of 26.85 $(^{\circ} C)$.

Material :Emissivity ($\epsilon$) :
Aluminum commercial sheet0.09
Cast iron, turned and heated0.60 - 0.70
Copper, heated and covered with thick oxide layer0.78
Copper, polished0.023 - 0.052
Gold0.47
Gold, polished0.025
INCONEL X, oxidized0.71
Iron, polished0.14 - 0.38
Iron, rough ingot0.87 - 0.95
Cast iron, turned and heated0.60 - 0.70
Nickel, polished0.072
Platinum, polished plate0.054 - 0.104
Carbon/stainless steel0.7 - 0.8
Tin, unoxidized0.025

### Heat Flux and Heat Flow​

Heat Flux or thermal flux is a flow of energy per unit area per unit of time ($W/m^2$).

Heat Flow ($W$) is the integral value of Heat Flux.

These heat exchange definitions can be used if the energy value that is going through a specific area is known.