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v. the thermo physical properties of volume concentrations

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v.           
To determine the
pressure drop in each case.

               
iv.           
To study the
effect of volume concentrations of nanoparticles on percentage change in Overall
Heat Transfer Coefficient compared with base fluid.

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iii.           
To study the
effect of volume concentrations of nanoparticles on Nusselt Number and Reynolds
Number.

                 
ii.           
To determine the
thermo physical properties of volume concentrations of titanium dioxide
nanoparticles dispersed in 40:60 ratio of ethylene glycol-water mixture.

                   
i.           
To perform
numerical investigation on different concentration of nanofluids and water to
evaluate heat transfer characteristics.

The main objectives of
the investigation are as follows:

The primary objective
behind this project is to study the heat transfer characteristics of a double
pipe heat exchanger using varying concentrations of nanofluid and tube wall
inserts experimentally. The tube wall inserts used are twisted tapes with a
twist ratio of 5 and the extended surfaces as triangular fins with 10 mm base
and 10 mm height and an arranged in staggered arrangement with a pitch of 30 mm
between two consecutive rows. The experiments were conducted using ? – Fe2O3
nanoparticle with base fluid mixture of Water and Ethylene Glycol in a volume
proportion of 60:40. The volume concentrations of iron oxide nanoparticles used
were 0.05%, 0.1%, and 0.2%. The nanofluid was used as a hot fluid and the tap
water was used as cold fluid.

1.5  AIM OF THE PROJECT

 

In summary, one
step method has more advantages than two step method due to improved dispersion
stability. However, the residual reactants that are left in the nanofluids due
to incomplete reaction or stabilizers and the functionality of stabilizers at
higher temperatures are the drawback of this one step method.

Although
nanoparticles are ultrasonically dispersed in liquid using a bath or tip
sonicator with intermittent sonication time to control overheating of
nanofluids, this two- step preparation process produces significantly poor
dispersion quality. Because dispersion quality is poor, the conductivity of
nano fluid is very low. Therefore, the key to success in achieving significant
enhancement in the thermal properties of nanofluids is to produce and suspend
nearly mono dispersed or no-agglomerated nanoparticles in liquids.

If nanoparticles are produced in dry powder form, some
agglomeration of individual nanoparticles may occur due to strong attractive
van der Waals forces between the nanoparticles. This undesirable agglomeration
is a key issue in all technology involving nanopowders. Making nanofluids using
the two-step processes has remained a challenge because individual particles
quickly agglomerates before dispersion and nanoparticle agglomerates settle out
in the liquids. The well-dispersed, stable nanoparticle suspensions are
produced by fully separating nanoparticle agglomerates into individual
nanoparticles in a host liquid. In most nanofluids prepared by the two-step
process, the agglomerates are not fully separated so nanoparticles are only
partially dispersed.

Stable suspensions of nanoparticles in conventional heat transfer
fluids are produced by two methods: the two-step technique and the single-step
technique. The two-step method first makes nanoparticles using one of the
above-described nanoparticle processing techniques and then disperses them into
the base fluids. The single-step direct evaporation method simultaneously makes
and disperses the nanoparticles directly into the base fluids. In either case,
a well-mixed and uniformly dispersed nanofluid is needed for successful
reproduction of properties and interpretation of experimental data. Most of the
nanofluids containing oxide nanoparticles and carbon nanotubes reported in the
open literature are produced by the two-step process.

Dispersion of Nanoparticles in base fluids

Chemical methods include chemical vapour deposition (CVD) method,
chemical precipitation, micro emulsions, thermal spray, and spray pyrolysis.
These nano-sized are most commonly produced in the form of powders. In powder
form, nanoparticles are dispersed in aqueous or organic host liquids for
specific applications.

Fabrication of
nanoparticles can be classified into two broad categories: physical processes
and chemical processes (Kimoto et al., 1963; Granqvist and Buhrman, 1976;
Gleiter, 1989). Currently a number of methods exist for the manufacture of
nanoparticles. Typical physical methods include inert-gas-condensation (IGC)
technique developed by Granqvist and Buhrman (1976.), and mechanical grinding
method.

1.4.2       
Method of making nanoparticles

Figure
– Schematic
representation of the multivariability of a nanofluid system.

Figure – Thermal conductivity of materials

The basic concept of dispersing solids in fluids to
enhance thermal conductivity is not new, and it can be traced back to Maxwell.
Solid particles are added because as shown the figure X, they conduct heat much
better than liquids. But the major problem is the rapid settling of these
particles in the fluids. Other problems are abrasion and clogging, which
seriously damage the application devices. Nanofluids have overcome these
problems by forming stable suspensions and also by lasting for longer duration
than millimetre or micrometre sized particles. The surface to volume ratio of
nanoparticles is thousand times larger than that of micro particles. The high
surface area of nanoparticles enhances the heat conduction of the nanofluids
since heat transfer occurs on the surface of the nanoparticles. The number of
atoms present on the surface of nanoparticles is very high as compared to
interior. Thus, this unique property results in higher stability and higher
thermal conductivity compared to other suspensions. Further, since
nanoparticles are small, they may reduce erosion and clogging, thus also
decreasing demand for pumping power.

1.4.1       
Importance of Nanosize

 

Despite
considerable previous research and development efforts on heat transfer
enhancement major improvements have been constrained because of the low thermal
conductivity of conventional heat transfer fluids. However, it is well known
that at room temperature, metals in solid form have orders-of-magnitude higher
thermal conductivities than those of fluids. For example, the thermal
conductivity of copper at room temperature is about 700 times greater than that
of water and about 3000 times greater than that of engine oil. The thermal
conductivity of metallic liquids is much greater than that of non-metallic
liquids. Therefore, the thermal conductivities of fluids that contain suspended
solid metallic particles could be expected to be significantly higher than
those of conventional heat transfer fluids.

1.4  CONCEPT OF NANOFLUID

 

When any two or more of these techniques are employed
simultaneously to obtain enhancement in heat transfer that is greater than that
produced by either of them when used individually, is termed as a compound
enhancement. This technique involves complex design and hence has limited
applications.

1.3.3       
Compound
Techniques

7.     
Additives for liquids include solid particles or gas bubbles
in single-phase flows and liquid trace additives for boiling systems.

6.    
Coiled tubes: Mostly used more in compact heat exchangers.
Secondary flow in the coiled tube produces higher single-phase coefficients and
improvement in most boiling regimes. However, a quite small coil diameter is
required to obtain moderate enhancement.

5.    
Swirl flow: These devices include a number of geometrical
arrangements or tube inserts for forced flow that create rotating or secondary
flow. Such devices include full-length twisted-tape inserts, or inlet vortex
generators, and axial core inserts with a screw-type winding. There are also
flow invertor or static mixer intended for laminar flows. They alternately
swirl the flow in clockwise and counter clockwise directions.

4.    
Displaced insert devices are devices inserted into the flow
channel to improve energy transport at the heated surface indirectly. They are
used with single- and two-phase flows. These inserts devices mix the main flow,
in addition to that in the wall region. The wire coil insert is placed at the
edge of the boundary layer, and is intended to promote mixing within the
boundary layer, without significantly affecting the main flow.

3.    
Rough surfaces: They may be either integral to the base
surface, or made by placing a “roughness” adjacent to the surface.
Integral roughness is formed by machining, or “restructuring” the
surface. For single-phase flow, the configuration is generally chosen to
promote mixing in the boundary layer near the surface, rather than to increase
the heat transfer surface area. The surface structuring forms artificial
nucleation sites, which provide much higher performance than a plain surface. A
wire coil insert is an example of a non-integral roughness.

2.    
Coated surfaces: These involve metallic or nonmetallic
coating of the surface. Examples include a hydrophilic coating that promotes
condensate drainage on evaporator fins, which reduces the wet air pressure
drop, or a non-wetting coating, such as Teflon, to promote dropwise
condensation.

1.      
Extended surfaces: These are routinely employed in many heat
exchangers. They provide effective heat transfer enlargement. The newer
developments have led to modified finned surfaces that also tend to improve the
heat transfer coefficients by disturbing the flow field in addition to
increasing the surface area. In extended surfaces or fin, use of a plain fin
may provide only area increase. However, formation of a special shape extended
surface may also provide increased h. Current heat transfer enhancement efforts
for gases are directed toward extended surfaces that provide a higher heat
transfer coefficient than that of a plain fin design. These surfaces involve
repeated formation and destruction of thin thermal boundary layers. Extended
surfaces for liquids use much smaller fin heights than those used for gases.
Shorter fin heights are used for liquids, because liquids typically have higher
heat transfer coefficients than gases and other reason is lower operating
pressure. Use of high fins with liquids would result in low fin efficiency,
poor material utilization and higher operating pressure.

1.3.2       
Passive
Techniques

6.      Jet
impingement forces a single-phase fluid normally or obliquely toward the
surface. Single or multiple jets may be used, and boiling is possible with
liquids.

5.      Injection
is utilized by supplying gas through a porous heat transfer surface to a flow
of liquid or by injecting the same liquid upstream of the heat transfer
section. The injected gas augments single-phase flow. Surface degassing of
liquids may produce similar effects.

4.     
Electrostatic fields can be in the form of
electric or magnetic fields or a combination of the two from dc or ac sources,
which can be applied in heat exchange systems involving dielectric fluids.
Depending on the application, it can also
produce greater bulk mixing and induce forced convection or electromagnetic
pumping to enhance heat transfer.

3.      Fluid
vibration are primarily used in single phase flows and are considered to be
perhaps the most practical type of vibration enhancement technique. The
vibrations range from pulsations of about 1 Hz to ultrasound. Single-phase
fluids are of primary concern.

2.      Surface
vibration at either low or high frequency has been used primarily to improve
single-phase heat transfer. A piezoelectric device may be used to vibrate a
surface and impinge small droplets onto a heated surface to promote “spray
cooling.”

1.      Mechanical
aids involve stirring the fluid by mechanical means or rotating the surface.
Mechanical surface scrapers, widely used for viscous liquids in the chemical
process industry, can be applied to duct flow of gases. Equipment with rotating
heat exchanger ducts is found in commercial practice.

In
these cases, external power is used to facilitate the desired flow modification
and the concomitant improvement in the rate of heat transfer. Augmentation of
heat transfer by this method can be achieved by following methods.

1.3.1       
Active
Techniques

3.      Compound
Techniques

2.      Passive
Techniques

1.      Active
Techniques

They
are broadly classified into three categories:

1.3  THE ENHANCEMENT TECHNIQUES

 

Not only are heat
exchangers often used in the process, power, petroleum, transportation, air-conditioning,
refrigeration, cryogenic, heat recovery, alternative fuel, and manufacturing
industries, they also serve as key components of many industrial products available
in the marketplace. These heat exchangers can be classified in many different
ways. We will classify them according to transfer processes, a number of
fluids, and heat transfer mechanisms. Conventional heat exchangers are further
classified according to construction type and flow arrangements. Another
arbitrary classification can be made, based on the heat transfer surface
area/volume ratio, into compact and non-compact heat exchangers. This
classification is made because the type of equipment, fields of applications,
and design techniques generally differ. All these classifications are
summarized in Fig.

The heat transfer surface
is a surface of the exchanger core that is in direct contact with fluids and
through which heat is transferred by conduction. That portion of the surface
that is in direct contact with both the hot and cold fluids and transfers heat between
them is referred to as the primary or direct surface. To increase the heat
transfer area, appendages may be intimately connected to the primary surface to
provide an extended, secondary, or indirect surface. These extended surface
elements are referred to as fins. Thus, heat is conducted through the fin and
convected (and/or radiated) from the fin (through the surface area) to the
surrounding fluid or vice versa, depending on whether the fin is being cooled
or heated. As a result, the addition of fins to the primary surface reduces the
thermal resistance on that side and thereby increases the total heat transfer
from the surface for the same temperature difference. Fins may form flow passages
for the individual fluids but do not separate the two (or more) fluids of the exchanger.
These secondary surfaces or fins may also be introduced primarily for
structural strength purposes or to provide thorough mixing of a highly viscous
liquid.

A heat exchanger consists
of heat transfer elements such as a core or matrix containing the heat transfer
surface, and fluid distribution elements such as headers, manifolds, tanks,
inlet and outlet nozzles or pipes, or seals. Usually, there are no moving parts
in a heat exchanger; however, there are exceptions, such as a rotary
regenerative exchanger (in which the matrix is mechanically driven to rotate at
some design speed) or a scraped surface heat exchanger.

Combustion and the chemical
reaction may take place within the exchanger, such as in boilers, fired
heaters, and fluidized-bed exchangers. Mechanical devices may be used in some
exchangers such as in scraped surface exchangers, agitated vessels, and stirred
tank reactors. Heat transfer in the separating wall of a recuperator generally
takes place by conduction. However, in a heat pipe heat exchanger, the heat
pipe not only acts as a separating wall, but also facilitates the transfer of
heat by condensation, evaporation, and conduction of the working fluid inside
the heat pipe. In general, if the fluids are immiscible, the separating wall
may be eliminated, and the interface between the fluids replaces a heat
transfer surface, as in a direct-contact heat exchanger.

Figure
– Classification of heat exchangers

A heat exchanger is a
device that is used to transfer thermal energy between two or more fluids,
between a solid surface and a fluid, or between solid particulates and a fluid,
at different temperatures and in thermal contact. In heat exchangers, there are
usually no external heat and work interactions. The general applications
involve heating or cooling of a working fluid and evaporation or condensation
of single- or multicomponent fluid streams. In other applications, the aim can
be to recover or reject heat or sterilize, pasteurize, fractionate,
concentrate, crystallize, or control a process fluid. In a few heat exchangers,
the fluids exchanging heat are in direct contact with each other. In most heat
exchangers, heat transfer between fluids takes place through a separating wall
or into and out of a wall in a transient manner. In many heat exchangers, the
fluids are separated by a heat transfer surface, and ideally, they do not mix
or leak. Such heat exchangers are referred to as direct transfer type, or
simply recuperators. In contrast, exchangers in which there is intermittent
heat exchange between the hot and cold fluids — via thermal energy storage and
release through the exchanger surface or matrix — are referred to as indirect
transfer type, or simply regenerators. Such exchangers usually have fluid
leakage from one fluid stream to the other, due to pressure differences and
matrix rotation/valve switching. Common examples of heat exchangers are shell and
tube exchangers, automobile radiators, condensers, evaporators, air preheaters,
and cooling towers. If no phase change occurs in any of the fluids in the exchanger,
it is sometimes referred to as a sensible heat exchanger.

1.2   
HEAT
EXCHANGER

 

Heat
transfer (or heat) is thermal energy in transit due to a spatial temperature
difference. Heat Exchanger is a device used to implement the process of heat
exchange between two fluids that are at different temperatures and separated by
a solid wall. The
subject of enhanced heat transfer has developed to the stage that it is of
serious interest for heat exchanger application. The refrigeration and
automotive industries routinely use enhanced surfaces in their heat exchangers.
The process industry is aggressively working to incorporate enhanced heat
transfer surfaces in its heat exchangers.

1.1   
BACKGROUND

INTRODUCTION

CHAPTER – 1

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