The process of heat transfer

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Many industrial processes involve the heat transfer. Heat transfer is a process by which internal energy from one substance transfers to another substance. The heat transfer occurs when existing of temperature difference between two regions. The heat transfer is normally occurs from a high temperature region to a lower temperature region. There are three modes of heat transfer. There are conduction, convention and radiation. The conduction mode is the transfer of heat in solids or fluids at rest conditions. The convention mode is occurs at fluids near a solid surface. While the radiation mode is the transfer of heat through the electromagnetic waves without medium required. The processes of heat tranfer cover a large range of temperatures and pressure. There are devices to perform heat transfers process called heat exchangers.

Heat exchangers are devices that conducting heat transfer between two fluids at difference temperatures (Kirk, 1999). In many types of heat exchanger, the two fluids are separated by a solid wall to prevent they are not in direct contact with each other. The primary heat transfer modes in heat exchangers are convention and conduction. The convention occurs in the boundary layer between the fluids on each side of the solid wall and the conduction occurs in the solid wall itself. Double pipe heat exchanger is one of the examples of heat exchanger applications. To maximize conduction heat transfer, the heat exchanger devices such as double pipe heat exchanger use liquid coolants to tranfer the heat. Thus, heat exchanger performance depends on the effectiveness of the heat tranfer fluids.

The heat transfer fluids play an important role in the development of heat exchanger equipment. However, the conventional coolant fluids such as ethylene, water and oil offer low heat transfer properties. Thus, the advanced heat transfer fluids development with higher heat transfer properties is in a strong demand. Therefore, to obtain higher heat transfer properties, a new generation of heat transfer fluids have been developed which is called nanofluid.

2.2) Double pipe

Double pipe is the one of heat exchanger applications. Double pipe heat exchanger is made of concentric inner and outer pipe (Sadik et al, 2002). Cold and hot liquid respectively flows in the gap of inner pipe and exchanges heat at the same time. U bends are used for inter connection inner pipe. The structure of double pipe heat exchanger is simple and heat transmission is large.

Figure 1: Double pipe heat exchanger in industry (www.brighthub.com)

Figure 2: the structure of double pipe heat exchanger (www.wikimedia.org)

The double pipe heat exchanger can be arranged as in figure 2. There are two shells joined at one end through return bend housing. In the inside of return bend housing, the central tube is bent or welded into a U-shape with the U-bend for this configuration. It will cause the shell side fluid to flow in series through each of two shells. The housing has a removable cover to allow removal or inner tubes.

The double pipe heat exchanger is also known as hairpin heat exchanger. These hairpins are based on modular principles. They can arrange in series (Figure 3), parallel (Figure 4) or series parallel combinations to achieve the duty of application.

Figure 3: An oil cooler with two hairpin sections arranged in series. (Sadik et al, 2002)

Figure 4: An oil cooler with two double pipe units in series on the shell side and parallel on the tube side (Sadik et al, 2002).

2.2.1) Purpose of double pipe heat exchanger

Generally, the double pipe heat exchanger used for high flow rate, temperature and pressure conditions. The heat transfer rate is high. The double pipe heat exchanger was developed to fit applications that are too small area and sections. They can be used in fouling conditions for easy to clean and maintenance. When small heat transfer areas are required, double pipe heat exchanger is excellent for heating and cooling fluids process. They are also very suitable for the fluids at high pressure due to the smaller diameter of the pipes.

2.2.2) Type of double pipe heat exchanger

The double pipe heat exchanger is made by joining tubes (Sarit, 2005). The design constructions produce difference types of double pipe units. The types of double pipe heat exchangers are listed in Figure 5.

Double Pipe Heat Exchanger

Two tubes unit

Multi-tubes unit

Multipass

U-tube

Single pass

Parallel-series

Series-series

Parallel-parallel

Figure 5: Types of double pipe heat exchanger (Sarit, 2005).

There are the details types of double pipe heat exchanger (Sarit, 2005):

Single pass unit

This is the simplest type of the double pipe heat exchanger. It made by inserting one tube into another pipe and sealing properly. This unit is not very popular because of relatively large length for a given duty and required large space even though the maintenance is easier. Usually, this type is used in small application such as inline heating of furnace oil for small domestic boiler. Figure 6 shows one of examples for single pass unit.

U-tube unit

This is the most popular design of double pipe heat exchanger. Figure 7 shows a U-tube unit. It is arranged by two inner tube are jointed at the end pipe with U-bend by welding. The two outer tubes are joined by a return bend housing. This type is allowed thermal expansion since the end of the pipe is joined with U-bend.

Multi-tube unit

Multi-tube double pipe are similar to the U-tube but different in using one inner pipe to a bundle of tube. For low pressure application, the tube sheet is sealed both on the shell side fluids. This type is suitable for low pressure and non-hazardous units. This type is shown in Figure 8.

Figure 6: A Single pass double pipe heat exchanger

Figure 7: A U-tube double pipe heat exchanger

Figure 8: Multi tube double pipe heat exchanger

2.2.3) The advantages of double pipe heat exchanger

There are the advantages of double pipe heat exchanger (Hewit et al, 1994):

They are cheap and readily available due to simplicity construction in its typical applications.

Thermal expansion did not occurs because of the U-tube type of construction

Easy of maintenances since shell circuit can be inspected and steam or mechanically cleaned due to simple construction and flanged joint.

They are flexible in any condition because come in with multi-pass tube circuit arrangement that can be adjusted either add up the joining or remove it.

Capable of withstanding thermal applications and support in high temperature and pressure applications.

They allow countercurrent heat exchange in which the cold fluids can be heated to a higher temperature since there are the hot fluids at the exit.

2.2.4) The applications of double pipe heat exchanger.

There are several applications for double pipe heat exchanger (Guy, 1978):

Oil and gas production.

Engine and transmission oil coolers.

Chemical and water heating applications.

Steam to liquid applications.

Marine fuel cooler.

In this project, the double pipe heat exchanger application in water heating for district heating facility is used to study.

2.3) Nanofluid

Nanofluid is the name conceived by Argonne National Laboratory to describe a fluid in which nanometre sized particles are suspended. The nanofluid is new kind of heat medium containing nanoparticles which is uniformly and stably distributed in a base fluid (Mohorianu et al, 2006). Nanofluid are engineered by suspending nanoparticles with average sizes below 100nm in common base fluids (Wenghua et al. 2007). Common base fluids are usually conventional coolants like water, oil and glycol. Figure 9 shows that the one example of nanofluid type, alumina, Al2O3 is smaller than 30 ± 5 nm.

Figure 9: Alumina, Al2O3 nanoparticles dispersed in water (Ji-Hwan Lee et al., 2007)

2.3.1) Production of nanofluid

The development of modern technology allows the fabrication of materials at the nanometre scale. Nanoparticles used in nanofluids have been made from many materials. The example of nanoparticles materials are oxide ceramics, nitride ceramics, carbide ceramics, metals and composite materials.

There are two techniques to produce nanofluids which are the two step technique and the single step technique (Wenhua et al. 2007). The two step technique starts with nanoparticles, produced by one of the physical or chemical synthesis techniques descried previously and then proceeds to disperse them into a base fluid. Meanwhile, the single step technique simultaneously makes and disperses the nanoparticles directly into the base fluid.

2.3.2) Nanofluid in enhancing thermal performance

Nanofluid is nanotechnology-based heat transfer fluids that are derived by stably suspending nanometre sized particles with typical length scales of 1 to 100nm in conventional heat transfer fluids. Research results from researchers show that nanofluids have thermal properties that are different from conventional heat transfer fluids.

For the high performance in thermal conductivity, several researchers have done some experimental and analysis study. For example, Masuda et al. (1993) has showed that the thermal conductivity of ultra fine suspensions of alumina, silica and other oxides in water increased by up to 30% for a volume fraction of 4.3%. Wang et al. (1999) reported that thermal conductivity for alumina and cupric oxide enhanced with a variety of base fluids including water and ethylene glycol. They observed a maximum of 12% increase in the conductivity with a volume fraction of 3% of alumina particles. On the other hand, the viscosity showed an increase of 20%-30% for the same volume fraction. Choi, et al. (2001) stated that the addition of a small amount of nanoparticles to conventional heat transfer liquids increased the thermal conductivity of the fluids up to approximately two times. Eastman et al (2001) found that the thermal conductivity of ethylene glycol nanofluid containing 0.3% volume fraction of copper particles can increased up to 40%. Pantzali et al. (2009) have conducted investigation the efficacy of nanofluids in plate heat exchanger. They found that when the nanofluid is added in coolant, the thermal conductivity is increased.

The convention heat transfer coefficient also has been studied and investigated by the researches. Choi (1995) reported a possibility to increase convention heat transfer coeffients is by using nanoparticles suspended in liquids. Byung et al. (2008) have conducted an experiment on effect of alumina in double pipe heat exchanger. They found that the addition of nanoparticles in the fluid increase the average heat transfer coefficient of the system in laminar flow. They also stated that the factors of enhancing the heat transfer of nanofluids are surface properties of nanoparticles, particles loading and particle shape. Leong et al. (2010) have investigated thermal performance of an automotive car radiator operated with nanofluids based coolants. They have concluded that with the increasing in volume concentration of nanofluids, the heat transfer rate is increased.

The researches have performed the investigation on the effect of nanofluids properties such as volume concentration to improve the thermal performance. Xuan and Li (2003) performed experimental study on Cu-water nanofluids up to 2% volume concentration and developed a Nusselt number correlation. They found that Cu-water dilute nanofluids have almost same pressure drop as water under same Reynolds number. Hwang et al. (2007) reported that thermal conductivity of the nanofluids depends on the volume fraction of particles and thermal conductivity of particles and base fluids. The thermal conductivity of aqueous alumina (Al2O3) nanofluids with low concentration has been investigated by Lee et al. (2008). This alumina (Al2O3) nanofluids produced by two step technique. They indicated that the thermal conductivity of aqueous nanofluids increases linearly with the addition of alumina particles.

Mintsa et al. (2009) researched the effect of temperature, particle size and volume fraction on thermal conductivity of water based nanofluids of copper oxide and alumina. They have suggested that to enhance the thermal conductivity was by increasing the particles volume fraction, temperature and particles size. When smaller particles are used in the same volume fraction, the contact surface area of particles with fluid and Brownian motion can be increased. Thus, this increased thermal conductivity of nanofluids. An experimental study on the heat transfer performance and pressure drop of TiO2-water nanofluids flowing in turbulent flow regime has conducted by Duangthongshuk and Wongwises (2010). They stated that the pressure drop of nanofluids was higher than the base fluid and increased with the increasing volume concentrations.

Table 2: The summary of literature studies in thermal performance operated with nanofluids.

Reference

Particles

Fluids

Findings

Masuda et al. (1993)

Al2O3, SiO2

Water

30% improvement for volume fraction of 4.3%.

Wang et al. (1999)

Al2O3, CuO

Water, EG

12% improvement for volume fraction of 3%

Choi, et al. (2001)

-

-

- approximately two times increment

Eastman et al (2001)

Cu

EG

40% improvement for 0.3% volume fraction

Pantzali et al. (2009)

-

-

Thermal conductivity increased

Choi (1995)

-

-

Convection heat transfer coefficient increase

Byung et al. (2008)

Al2O3

Average heat transfer coefficient increase

Leong et al. (2010)

Cu

EG

Heat transfer rate increased

Xuan and Li (2003)

Cu

Water

Pressure drop increased for 2% volume fraction

Hwang et al. (2007)

-

-

Increasing depending on volume fraction percentage

Lee et al. (2008).

Al2O3

Water

Thermal conductivity increased

Mintsa et al. (2009)

CuO, Al2O3

Water

Increasing depending on volume fraction percentage

Duangthongshuk and Wongwises (2010)

-

-

Increasing depending on volume fraction percentage

Note: EG: ethylene glycol

2.3.3) Thermal properties of nanofluids

The thermal conductivity of nanofluids was the main focus in the stages of nanofluids research. In any case, the properties of nanofluids depend on the thermal conductivity, specific heat, density and dynamic viscosity. Vasu et al. (2008) has listed some of the properties of nanoparticles and based fluids in Table 3. Pantzali et al. (2009) has systematically measured the thermophysical of the nanofluids. Some properties of nanofluids and base fluids are shown in Table 4.

Table 3: Thermophysical properties of nanofluids and base fluids (Vasu et al, 2008)

Property

Water

Ethylene

glycol

Cu

Al2O3

CuO

TiO2

C [J/kgK]

4179

2415

385

765

535.6

686.2

ρ [kg/m3]

997.1

1111

8933

3970

6500

4250

K [W/mK]

0.605

0.252

400

40

20

8.9538

α [m2/s]

1.47

93

1163

1317

57.45

30.7

Table 4: Measured thermophysical properties of water and nanofluids at 25ÌŠ c (Pantzali et al. 2009)

Index

Thermal conductivity, kn (W/mK)

Density

ρn (kg/m3)

Heat capacity cp,n (J/kgK)

Viscosity

µn (mPas)

Base fluid: water

0.607

998

4180

1.0

Al2O3-4

0.648

-

-

-

CuO-3

0.643

-

-

-

CNT

0.628

-

-

-

Al2O3-3

0.643

1050

3910

2.9

TiO2

0.643

1030

3960

2.0

CuO-8

0.720

1510

2700

5.6

CuO-4

0.669

1250

3280

2.0

CuO-2

0.631

1130

3720

1.3

2.3.4) The advantages of nanofluids

Since the solid in nano-sized particles (1-100nm) are suspended in the base fluid, it has higher thermal conductivity heat transfer coeffients compare to other conventional coolants. There are other advantages ( Choi, 1995):

Improved heat transfers

Heat transfer system size reduction

Minimal clogging

Miniaturization of the systems

2.3.5) Applications of nanofluids

In variety of thermal system, nanofluids can be used to improve heat transfer and energy efficiency. Many applications have been used nanofluids since the production companies see the potential of nanotechnology in industrial applications. The examples of application are ( Wenghua et al. 2007):

Transportation sectors

The nanofluid is added up in automotive engine coolant to increase the thermal conductivity.

The nanofluid has been applied to the cooling of automatic transmissions to improve heat transfers from transmission cooling (Tzeng et al. 2005).

In automotive lubricants applications, nanoparticles stably dispersed in mineral oil to reduce wear and enhancing load carrying capacity (Zhang and Que. 1997).

Electronics cooling industry

Nanofluid has been used as working fluid for heat pipes in electronic cooling applications (Tsai et al. 2004).

Nanofluids have been investigated on the heat capability of an oscillating heat pipe (Ma et al. 2006).

Defence development

Nanofluid have potential to provide cooling in military system like power electronic device, directed energy weapon, military vehicles, submarines and high power laser diode.

Space exploring

The power of density is very high makes nanofluids attractive in general electronic cooling.

The magnitude increase in the critical heat flux in pool boiling using nanofluids ( You et al. 2003 & Vassalo et al. 2004)

Nuclear systems cooling

The potential impacts of the use of nanofluids are on the safety, neutronic and economic performance of nuclear system.

Biomedicine

Iron based nanoparticles used as delivery vehicles for drugs or radiations without damaging nearby healthy tissue during cancer treatment.

Nanofluids could also used for safer surgery by producing effective cooling around the surgical region.

In contrasting application to cooling, nanofluids could be used to produce a high temperature around tumors to kill cancer cells without affecting nearby healthy cells (Jordan et al. 1999).

Other applications

In buildings where increases in energy efficiency could be realized without increased pumping power.

Nanofluids coolants also have potential application in major process industries such as materials, chemicals, food and drink, paper, textiles and etc.

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