“To understand the mechanism of heat transfer in solid and fluids”
Aim of this task is to understand the mechanism behind the heat transfer in solid and fluid under control environment. Mechanism of heat transfer inside solid material, between two different solid materials and between solid and fluid will be discussed in this lab work.
In order to understand the mechanism of heat transfer in solid and fluids the below mention objectives should be completed fully in said sequence.
- Understand the basic of heat transfer in solid and fluid using the mechanism of conduction and convection
- Understand the mechanism of forced convection at the interface of the fluid and solid
- Understand the basic of energy balance in the system of heat transfer
- Perform experiment to analyze the force heat transfer and energy balance when air flow inside heated tube
The process of heat transfer in solid material happen due to the mechanism of heat transfer called conduction. Where heat transfer from the region of high potential to the region of low potential happened inside the material and it depends on the material ability to transfer heat from its one side to another side. The ability of material to transfer heat from its side of high potential to the side of low potential is called thermal conductivity of material or in the case of composite wall where wall is made of more than one material it is called the heat transfer coefficient.
Heat transferred due to conduction can be calculated as
Q is the heat transferred due to conduction in Watt
k is the material thermal conductance W/mC
x is the thickness of the wall across which heat will be transferred in meter m
A is cross section area of wall in square meter m^2
∆T is temperature difference between two sides of wall in degree centigrade C
The process of heat transfer at solid and fluid interface happen due to the mechanism of heat transfer called convection. Where heat transfer from the region of high potential to the region of low potential happened at the solid and fluid interface and it depends on the material and fluid ability to transfer. This ability to transfer heat at the solid and fluid interface is called the convective heat transfer coefficient and heat transfer in this case can be calculated as
Q is convection heat transferred in Watt w
h is the convection heat transfer coefficient W/mC
∆T is temperature difference between solid and fluid in degree centigrade
The mechanism of the heat transfer at the solid fluid interface under a control environment is called force convection. In this mechanism fluid is given a specific flow rate to control the amount of heat transfer to the fluid from solid or from solid to fluid. Force convection depends on the temperature different, mass flow rate of the fluid and fluid heat capacity. Force convection can be calculated as follow
Q= m* C_p*( T_o- T_i )
Q is the heat transfer in Watt
m_h is fluid mass flow rate in Kg/sec
C_ph is fluid specific heat in J/Kg.C
T_ho is fluid outlet temperature in degree centigrade C
T_hi is fluid inlet temperature in degree centigrade C
In thermodynamic system where heat transfer is happening between a source and sink present in thermodynamic system, according to the laws of thermodynamics the all heat provided by the source cannot be converted into useful work or transfer to the other material. Some the heat provided by the source is wasted into surrounding or in flaw present in the system. To understand this process where a fixed amount of heat is transfer from source to sink concept of energy balance used. Below is the explanation of energy balance of the thermodynamic system.
For an ideal case
Q_(provided )= Q_(transfer )
For practical case
It can also be written as
Q_(tansfer )= Q_provided-Q_(loss )
The apparatus to study the force convection of fluid consist of an electrically controlled centrifugal Fan whose main function in to through the air into the system at required flow rate. The system consists of inlet pipe which take air from centrifugal fan and pass it to a copper tube. Copper tube has an orifice setup attached to it to measure the flow rate of the air inside the tube. Copper tube is heated using a metal coil which gets its power from an electrical setup. Copper tube and the coil around its outer surface are insulated from the outer atmosphere using an insulation material. There thirteen thermocouples attached to the system of copper tube and insulation around it to record the temperature of the system. Some of the thermal properties involve in the work are as follow
- Copper thermal conductivity 380 W/m C
- Lagging thermal conductivity 0.04 W/m C
- Air specific heat capacity 1005 J/kg C
- Air average density 1000 Kg/m^3
- Orifice plate discharge coefficient 0.613
The experimental procedure of the force convection using the above mention setup is explained below.
- Setup the apparatus for the experiment that is connect the electrical connection, ensure all thermocouples are attached properly
- After the complete setup, start the apparatus and wait until the steady stage condition is achieved
- After achieving the steady state condition,
- Note the value of voltage and current of the heater
- Note down the air inlet temperature
- Measure the value of temperature from all thermocouples
- Record the height of column of manometer of orifice tube
- Move the pilot tube up and down in duct to measure temperature at different positions
Follow are the values measured during the experiment of force convection
Number Temperature C
Position on Scale mm T 14
As shown in the above calculation section the heat input by the electrical system is about 760 W which moves toward the copper tube and lagging material. In ideal case the lagging material should not absorb any heat from the system but it does in practical cases. The copper tube absorbs heat and transfers it to the air. Air absorbs the amount of heat equal to 351 watt which is less than the 50 % of the heat produce by the electrical system. Losses happening in the system are the heat lost by the radial heat transfer in the lagging material and based on the calculation is it just 40 watt for the entire length of the lagging material. The losses are also happening along the length by the cross section of tube and lagging material and based on the calculation their value are almost negligible. Reason for very small about of the heat lose in longitudinal is that the major heat flow is in radial direction as heat source is around the outer diameter of the copper tube. Other than this very small cross section area of copper tube and very bad thermal conductivity of lagging material and no special heat source or sink along the length of the of tube are the reason for very small longitudinal heat loss.
The energy imbalance show that the more than 50 percent of the heat generated by the electrical system is in loss and is also undocumented means the reason of loss is not known. This imbalance in the system can be considered as an error in experimental process which results in loss of more than 50 percent. The error in the experimental can be wrong approach to measure the heat loss as heat transfer by the system in terms of loss is being measured but the heat absorb in the system or the heat wasted due to defect in the system is not being measured at all. The heat absorb by the copper tube which raise its temperature and similarly the heat absorb by the lagging material which raise its temperature are not documented. Other source of error are the faulty apparatus where the heat which electric should is generating is less than the calculated value of the source and similarly the heat generated by the coil, heat transfer by copper tube or the lagging material can have defects.
The aim of understanding the heat transfer at the solid fluid interface due to force convection has been completed successfully as the heat generated, transferred and loss were calculated. The energy imbalance in system was measured and reason for imbalance like heat absorb by material, improper methods of measuring or faulty apparatus were discussed. The calculation show the less than 50 percent of the heat is transferred to the air were less very is amount is available in the form of known losses. The energy imbalance is perfectly makes sense as the assumptions of uniform heat transfer, perfect material insulation or very efficient heat transfer by copper tube are made before the experiment. The heat which will be absorbs in the system for example heat absorb by the copper tube, heat wasted inside the copper tube and lagging face is not being measured. Other than this in the longitudinal heat transfer the flow of heat which is considered long length of the tube is not proper as no heat source is available for heat flow in that direction.