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 surface	10
Moderate speed flow of air over a surface	100
Moderate flow of water in a pipe	3000
Water and liquids	50 to 10'000

Natural convection :	$W/m^2K$ :
Gases and dry vapors	5 to 37
Gas flow in tubes	10 to 350
Water and liquids	50 to 3000
Air	10 to 100
Water	100 to 1200
Water flowing in tubes	500 to 1200
Oil	50 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. $(W/m^2K)$ :
33	2975
59	3828
99	7972
149	10004
198	11264
245	11832
296	13011
348	14799
398	17155
447	17887
499	18415
544	18049
598	17115
646	16099
699	13823
749	12076
798	10045
848	7566
898	5088

Radiation

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 sheet	0.09
Cast iron, turned and heated	0.60 - 0.70
Copper, heated and covered with thick oxide layer	0.78
Copper, polished	0.023 - 0.052
Gold	0.47
Gold, polished	0.025
INCONEL X, oxidized	0.71
Iron, polished	0.14 - 0.38
Iron, rough ingot	0.87 - 0.95
Cast iron, turned and heated	0.60 - 0.70
Nickel, polished	0.072
Platinum, polished plate	0.054 - 0.104
Carbon/stainless steel	0.7 - 0.8
Tin, unoxidized	0.025

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.