 ## Fluids

#### I want to find the real time for heating water in a tank by a heat source of 18500 watt /cubic meter

• hsienfahs
Subscriber

dear everyone ,

i am beginner in CFD and a have a problem in finding the exact real time for heating water in the tank below by a source term of 18500 w/cubic meter. I am working stationary simulation with no fluid flow. Briefly i want to find the graph showing the real increase of temperature as function of time ( the chart below shows that the time in (s) is very big number )

• DrAmine
Ansys Employee
But you have already that graph showing the temperture vs Time? Can you please clarify a bit more and what do you want to do?.If you want to ge the time to heat up a volume of water in closed domain to certain temperature I think no CFD is required.n
• hsienfahs
Subscriber
thanks DrAmine ; the graph shows that the time it tooks to heat water is illogical, i want to know what is this time in the x-axis n
• hsienfahs
Subscriber
7000000 s is an illogical number of time to heat a small amount of water ( volume = 0.16 cubic meter ) to a temperature 345 K n
• Rob
Ansys Employee
Can you check the domain size and give us some details on the boundary conditions? n
• hsienfahs
Subscriber
yes Mr. Rob ni have a cylindrical shape tank of diameter 0.38 meter and Height 0.64 meter made from steel as a cover with thickness 0.02 m ( insulated ) , a heater( titanium ) inside it of length 0.13 m and diameter 0.03 m with a source of 18500 watt/cubic meter and the temperature of water initially is 298 K should be heated to reach 245-250 K. The mesh i took is by default.nplease advise me n
• Rob
Ansys Employee
You're adding the energy to the whole volume? What are the external boundary settings?n
• hsienfahs
Subscriber
the resistor i put at the middle bottom of the tank , the source term is a constant heat generation 18500 watt/ meter cube nand there is no heat flux or anything in the cover wall ( all values are zero )nthis heater should heat all the volume in the tank
• hsienfahs
Subscriber
• Rob
Ansys Employee
So, given the size of a resistor you're adding a few watts into the domain? n
• hsienfahs
Subscriber
we have given 1200 w resistor nto heat a volume of 0.06 meter cube
• hsienfahs
Subscriber
what is the problem or error in my work n
• DrAmine
Ansys Employee
Your time step is 2000 [s]. So the Time appearing on your x-axis is logical.n
• hsienfahs
Subscriber
is this time appearing is the real time for heating ? what it shows ?ni need the real time of heating this water in this tank n
• DrAmine
Ansys Employee
You are using it: that is your input. Actually very high value to really capture heating up in Fluid regions. By the way are you doing FSI?nRegarding the source term: if you want to add 1200 W into resistor volume you define the source term as 1200 W / Volume of Resistor.n
• hsienfahs
Subscriber
no i am using CFD nin this case the source term will be 13,000,000 w/meter cube , since the diameter of resistor is 0.03 m and the length is 0.13 m and the power needed is 1200 w ni have a problem to determine the time step size and number of time step. Knowing that experimentally in an electrical water heater the time needed for heating is 3 hours to reach temperature of 80 C n
• Rob
Ansys Employee
The CFD time step needs to be suitable to capture the flow features, so will typically be under 1 second. n
• DrAmine
Ansys Employee
Moreover pay attention for your grid and try to resolved the fluid side of the resistor walls: here where the heat is transfer from the solid domain to the fluid domain. Also think about accounting for buoyancy effects by at least having a Boussinesq approximation for your water (temperature dependent density or simple constant but only the buoyancy term is there).
• hsienfahs
Subscriber
thanks for previous answers ni am running my simulation as transient, the temperature at first is raising in very good way them it become so slow in increasing what is my missing or wrong work.nplease advise
• Rob
Ansys Employee
What is the water density?n 