Mechanism of Heat Transfer in Solid and Fluids

Aim 

“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.

Objectives

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

Theory 

Conduction

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=k/x*A*∆T

Where

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

Convection

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=h*A*∆T

Where

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

Force Convection

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

Energy Balance

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

Q_(provided )=Q_transfer+Q_lost  

It can also be written as

Q_(tansfer )= Q_provided-Q_(loss )

Experimental work

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

Measurements

Follow are the values measured during the experiment of force convection

Number Temperature C

1 56.7

2 60.9

3 63.1

4 66.2

5 70.6

6 65.3

7 64.5

8 61.1

9 37.3

10 68.5

11 40.0

12 37.7

13 45.3

Position on Scale mm T 14

141.5 45.6

149 39.2

156.5 38.8

164 41.7

171.5 49

Discussion

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. 

Conclusion

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.

Organic Fluids and Rankine Cycle (selection process for organic fluid)

In organic Rankine cycle, an organic fluid is used in the place of water to reduce the cost of heating done in the boiler. The organic fluid used has a lower boiling point as compared to water due to which it requires less heat in the boiler to produce the organic fluid steam and it also requires less energy at condenser for cooling (S. Quoilin 2011). Less heat requirement means low fuel-burning which reduces the cost of operating the Rankine cycle. The organic fluid selected for the organic Rankine cycle is evaluated on strict selection criteria and the fluid performing the best is select as working fluid. The selection criteria of the organic fluid consist of thermal properties that influence the performance of organic fluid or are very critical for the basic working of the organic Rankine cycle. 

Following is the list of thermal properties and why they are important for the evaluation of the organic fluid of organic Rankine cycle

1. Heat Capacity

The heat capacity of the fluid is defined as the amount of heat required to increase the temperature of the fluid by one degree.  It is measured as joule per kelvin or centigrade. It’s a critical thermal property of the fluid in this case as a material with high heat capacity will require more heat to increase its temperature and at the same time will deliver more heat for a decrease in its temperature.

2. Thermal Conductivity

Fluid ability to transfer heat rather it is moving into the fluid or moving out of the fluid. Fluid for ORC should have a high thermal conductivity in order to make the heat transfer process at the heat exchanger or boiler efficient. 

3. Viscosity

The viscosity of the fluid defines the resistance made by the fluid to change the state of rest or continuous motion of one molecule with respect to the other. The viscosity of the organic fluid should be low for the ORC case so the pump does not work too hard.

4. Chemical Stability

Chemical stability of the organic fluid used in the organic Rankine cycle is also the thermal stability of the fluid. The chemical stability of the organic fluid of the organic Rankine cycle means the ability of the fluid to resist decomposition, deterioration, and any chemical reaction with container or tube material at high temperature during the process and its useful life. The chemical stability of the organic fluid of the organic Rankine cycle has to be excellent because organic fluid with low chemical stability will decompose and react inside the organic Rankine cycle forcing the system to stop or reduced performance. 

5. Low Freezing Point

Long with its ability to resist high temperatures the ability of organic fluid of organic Rankine cycle to resist freezing inside the turbine or condenser is also very important. The sudden expansion of organic fluid inside the turbine or condenser of the organic Rankine cycle can force the organic fluid to freeze thus forcing the system to stop. The freezing point of organic fluid of the organic Rankine cycle should be lower than any point in the complete organic Rankine cycle. 

6. Heat of Vaporization

The organic fluid used in an organic Rankine cycle should have the ability to absorb a high amount of energy before it gets evaporated. High latent heat of the organic fluid will help it to absorb more energy from the boiler of the organic Rankine cycle which will in return help to reduce the mass flow rate of the organic fluid of the organic Rankine cycle. This also helps to reduce the size of the pumping and related system of organic Rankine cycle thus saving the initial cost of installation of the organic Rankine cycle.

7. Density

The density of the material defines the material ability to hold heat and it defines the organic fluids' ability to deliver high quality and quantity of the heat. Greater the density of the organic fluid of the organic Rankine cycle greater will be the heat absorb, maintain, and delivered by the organic fluid or organic Rankine cycle.

8. Pressure

The organic fluid used for the organic Rankine cycle should be able to resist the pressure applied onto it during the working of the organic Rankine cycle. The organic fluid used for the organic Rankine cycle should not decompose or deterioration and any chemical reaction with container or tube material at high pressure.

9. Environmental aspect

The organic fluid used in an organic Rankine cycle should have minimum impact on the environment. The impact on the environment includes the energy and resources required to obtain the required organic fluid of the organic Rankine cycle. The environmental aspect also includes the decomposition and disposal of the organic fluid after its useful life.

10. Safety

The organic fluid used for the organic Rankine cycle should be safe to handle and safe to work with. The organic fluid used for organic Rankine cycle should not be harmful to human and surrounding in its natural state and at normal temperature

11. Availability

The organic fluid used for the organic Rankine cycle should be easily available as a good quantity of the fluid is required in the organic Rankine cycle. 

12. Cost

The cost of the organic fluid used for the organic Rankine cycle should below in order to keep the cost of operation of the organic Rankine cycle lower than standard Rankine Cycle.

Organic Fluids of Organic Rankine Cycle

Selection of the Organic fluid for use in the organic Rankine cycle is one of the most important steps in analysis and optimization of ORC. So the selection of the fluid is done on the basis of the work done in the literature and the fluid thermal properties mention above. Some of the organic fluids used in previous work are Iso-butane, n-pentane, Iso-pentane, methanol, ethanol, R123a, R245fa and dichloro-trifluoro-ethane (HCFC-123). Work of kamyar darvish on selecting the optimum fluid for organic Rankine cycle is show that the Iso butane and R245fa is much better as compared to n-pentane and Iso-pentane whereas the work of Yamamoto (1999) show that dichloro-trifluoro-ethane is better than methanol, ethanol, and water. Comparing the thermal and physical properties of dichloro-trifluoro-ethane, Isobutane and R245fa shows that the dichloro-trifluoro-ethane cannot perform better as it has very high density. Iso butane has lower boiling temperature and latent heat but R245 has higher density and high critical temperature.

Fluid	Density Kg/m^3	Boiling point K	Latent Heat KJ/Kg	Critical Temp K	Critical Pressure MPa	Freezing point K	Safe
R245fa	5.71	288.4	196	427	3 .65	167	Non flame able
Iso butane	2.44	261	165	407	3.64	114	Nonflame able

5 Eco Friendly Heat Energy Resources for Organic Rankine Cycle

Organic Rankine cycle ORC is one of the best options available for small scale efficient energy production systems. In Organic Rankine Cycle an organic fluid is used in the Rankine cycle instead of water as the organic fluid has a lower boiling point as compared to water so required less heat input. Waste heat from any source, when combined with organic Rankine cycle ORC using a heat exchanger like shell and tube heat exchanger, make it the best and most environmentally friendly energy solution. Optimize parameters allows the minimum mass flow rate and less area required for the heat exchanger saving power and cost required at the heat exchanger system without compromising the efficiency of the system. 

With a continuous increase in the world population, the problems associated with human needs are also increasing. One such problem is the need for energy to meet certain needs like electricity. In today’s industrial world where there is a complete working industry for every human need, every industry itself has a need and the biggest of them is electricity.  The world current setup to meet its need for electricity heavily depends on the use of natural resources which has increased to its maximum in the present time (Energy.gov, 2019). Natural resources like coal, gas, and fossil fuel on which humanity depends to meet their need for electricity have limited known stock due to which researchers and engineers are forces to developed new resources to produce electricity(Energy matter 2019). The use of natural resources like coal is also harmful to the earth's environment and the current use of this type of natural resource to produce electricity has polluted every land, river, sea, and even oceans. So there is a desire need to develop some renewable and alternative resources which can produce clean electricity. 

Figure 1 global energy consumption [(energy.matter, 2019).

 

Figure 2 energy consumption by resources (energy.gov, 2019).

There are some methods already developed like solar farms, wind turbine farms, geothermal sources, and hydroelectricity which use renewable natural resources like sunlight, wind, geothermal heat sources, and flowing water for electricity production (Energy matter 2019). All these resources do not harm the environment as coal or natural gas do and they all are also renewable but these resources have limitations of their now. The hydroelectric method is only possible in those locations that the huge amount of flowing water with the all necessary setup to store and control the flowing water. Solar farms need continuous sunlight for production which limits their use to an area with getting sunlight 12 months a year but still they only operate during the day timing. The wind farm covers a lot of surface area and they are also limited to geographic regions that have potential wind energy 12 months a year.

Contrary to this the conventional resources do not have regional limitations and product electricity 24/7 for 365 days a year but they pollute the environment and with limited natural resources world cannot rely on them for long. So there is a need to integrate conventional resources with renewable ones to increase electricity production without affecting the environment. One such way is to run steam power plants with renewable resources as one of the most commonly used and high energy production methods having quite good overall efficiency as compared to other methods of electricity production. Steam turbines operate on the Rankine cycle uses conventional methods to heat water and produce steam at the required temperature and pressure. An improved form of regular Rankine cycle is the Organic Rankine Cycle which uses an organic fluid in the place steam as its main working fluid. This research work optimizing the organic Rankine cycle with the help of renewable and alternative resources.

Hybrid Organic Rankine Cycle

In the hybrid organic Rankine cycle, a renewable and alternative source of heat is used in the place of a conventional boiler and fuel-burning systems. In one of the systems, a solar heater is used to heat the organic fluid and convert it into steam during the day time and biofuel is used to heat organic fluid at night. The solar heater collects solar heat at its collector and transfers it to a fluid that fluid carries that heat to an organic fluid of the Rankine cycle and heat the fluid until it converted into steam. Conversion of heat happens in a heat exchanger which also works as a biofuel burner during the night time to heat the organic fluid (S. Quoilin 2011).

 

Figure 3 Hybrid Rankine Cycle (S. Quoilin 2011)

Heat Source for Hybrid ORC

A conventional organic Rankine cycle makes use of conventional resources for the heat they required to vaporize the organic fluid inside the boiler stage of the Rankine cycle. The conventional resource of heating in the organic Rankine cycle is the burning of fossil fuel. To transfer the dependence from fossil fuel, the use of alternative and renewable fuel is needed for the organic Rankine cycle. 

There are a number of alternative resources available for the heating of organic fluid of organic Rankine cycle and they are explained as follows.

1. Waste Heat Recovery

One of the best, biggest, and most efficient sources of the heat for the organic Rankine cycle is the waste heat coming out of any heating source. This source of heat or organic Rankine cycle is considered best as it does not require any running cost of the organic Rankine cycle. The waste heating coming out of any industrial unit or domestic unit can be used as a heat source for the organic Rankine cycle and the fact that organic fluid of organic Rankine cycle needless amount of heat as compared to water for vaporization makes this waste heat source quite effective for organic Rankine cycle. Using waste heat recovery for the organic Rankine cycle also requires less initial investment as only a heat exchanger is required for extracting waste heat and transferring it to the organic fluid of the organic Rankine cycle.

2. Biomass 

Biomass as a heat source for the organic Rankine cycle is one of the simplest, cost-effective, and environmentally friendly sources. This limitation of biomass as a fuel is the low quantity of temperature obtains from it does not apply here as the organic fluid of the organic Rankine cycle requires less temperature or heat to get vaporize as compared to water. The biomass as a source of heating does not require the high cost of running the organic Rankine cycle as biomass is usually a waste product of a process that will decompose in nature if not utilized. Some of the advantages of utilizing the biomass as fuel in organic Rankine cycle are, it does not require any specific or expensive machine or set for heating, it is not restricted to any specific area or conditions, it does not have high operation cost and it is completely environment friendly. 

3. Geothermal

The geothermal source of heat for the organic Rankine cycle is considered effective and efficient as it clean, alternative, and renewable source of the heat of the organic Rankine cycle. This source of heat does not require any boiler cost of machinery but the setup required for the geothermal source of heat involves high initial cost. Geothermal source of heat in the organic Rankine cycle does not have high operation costs but has a high limit of the geographic location of the geothermal source. Geothermal source of heat in the organic Rankine cycle is very economical and is highly environment friendly.

4. Solar Heating

Using heat available in the sunlight to heat the organic fluid of the organic Rankine cycle is also an effective, efficient, renewable, and cost-effective solution. This alternative and renewable resource of heating require only a single solar collector which can focus sun rays onto a pipe containing specific fluid usually a refrigerant. The solar heating methods also required a heat exchanger to transfer solar heat absorbed by the refrigerant to the organic fluid of the organic Rankine cycle. This method of heating in the organic Rankine cycle does not require any fuel to operate, does not have a hard geographic limitation, and is super environment friendly but is only applicable during the day timing. Due to this limitation, an extra source of heat is always required in this method which also increases its cost of installation and operation.

The different energy sources which can be used to provide the heat required for the organic Rankine cycle include waste heat energy from industrial units, solar heat energy, geothermal energy, and biomass. From these available energy resources, the waste heat from the industrial unit is most favorable as it does not require any running cost of the organic Rankine cycle. Using waste heat recovery for the organic Rankine cycle also requires less initial investment as only a heat exchanger is required for extracting waste heat and transferring it to the organic fluid of the organic Rankine cycle. For this study waste heat coming out of the kiln of the cement factory where a lot of heat is wasted into the environment as the end of the process will be used. The fluid which contains heat moving out of the kiln and into the environment is simple heated air. Shell and tube heat exchanger with the optimum design parameters of the organic Rankine cycle was designed in such a method its required minimum mass flow rate of air which allows energy saving at the pump and cost-saving at the heat exchanger installation and maintenance without compromising the efficiency of the system.