Performance Evaluation of Heat Transfer And Friction Factor Characteristics Of Double Pipe Heat Exchanger Fitted With Square Jagged Twisted Tape Inserts

© 2015, IERJ All Rights Reserved Page 1

ISSN 2395-1621 Performance Evaluation of Heat

Transfer And Friction Factor

Characteristics Of Double Pipe Heat Exchanger Fitted With Square Jagged

Twisted Tape Inserts

#1

Satyajit D.Ghorpade,

Prof (Dr.). K.P.Kolhe

1[email protected]

2[email protected]

#1

PG Student: Mechanical Engineering Department

JSPM’s Imperial College of Engineering & Research Wagholi,Pune. (India)

#2

Professor: Mechanical Engineering Department

JSPM’s Imperial College of Engineering & Research Wagholi,Pune. (India)

ABSTRACT

ARTICLE INFO

In the present study, the characteristics of heat transfer parameter and achievement of pressure drop at a place of horizontal & double pipe with square jagged twisted tube inserts are investigated. As a working fluid cold and hot water are used in shell and tube side respectively. As a result of insertion of helical screw tape, square jagged twisted tape in a concentric double tube heat exchanger gives better effectiveness on a flow friction characteristics and heat transfer rate those are experimentally investigated. Mean fanning friction factor, heat transfer coefficients &

factors of thermal performance in tube fitted with square jagged twisted tape inserts are augmented respectively. The consideration is done with the effect of twist ratio and other related parameters on characteristics of heat transfer and pressure drop.

The major goal of enhancement of heat transfer rate is possible with encouragement of high heat fluxes. The application of square jagged twisted tape was found to improvement in condensing heat transfer coefficients. In addition the effectiveness of square jagged twisted tape on heat transfer enhancement is also investigated. The outcomes of experimental investigations are that combination of twisted tape inserts with square jagged performs significantly better than individual enhancement technique. The study gives better agreement with the introduction of turbulence flow which can be used to predict improved performance of heat transfer rate.

Keywords— Square tape inserts;Channel flows turbulators; Performance Criteria ; thermal performance; friction factor.

Article History

Received : 10^th June, 2015 Received in revised form : 10^th June, 2015

Accepted : 14^th June, 2015 Published online : 14^th June, 2015

I. INTRODUCTION

Heat exchangers are widely used in various industrial process for heating and cooling applications such as air conditioning and refrigeration systems, heat recovery processes, food and dairy processes, chemical process plants etc. The major challenge in designing a heat exchanger is to make the equipment compact and achieve a high heat transfer rate using minimum pumping power. Techniques for heat transfer augmentation are relevant to several engineering applications. In recent years, the high cost of energy and material has resulted in an increased effort aimed at producing more efficient heat exchange equipment.

Furthermore, sometimes there is a need for miniaturization

of a heat exchanger in specific applications, such as space application, through an augmentation of heat transfer. For example, a heat exchanger for an ocean thermal energy conversion (OTEC) plant requires a heat transfer surface area of the order of 10000 m²/MW [1]. Therefore, an increase in the efficiency of the heat exchanger through an augmentation technique may result in a considerable saving in the material cost. Furthermore, as a heat exchanger becomes older, the resistance to heat transfer increases owing to fouling or scaling. These problems are more common for heat exchangers used in marine applications and in chemical industries. In some specific applications, such as heat exchangers dealing with fluids of low thermal

© 2015, IERJ All Rights Reserved Page 2 conductivity (gases and oils) and desalination plants, there is

a need to increase the heat transfer rate. The heat transfer rate can be improved by introducing a disturbance in the fluid flow thereby breaking the viscous and thermal boundary layer. However, in the process pumping power may increase significantly and ultimately the pumping cost becomes high. Therefore, to achieve a desired heat transfer rate in an existing heat exchanger at an economic pumping power, several techniques have been proposed in recent years and are discussed under the classification section.

Classification of Augmentation Techniques:

They are broadly classified into three different categories:

1. Passive Techniques 2. Active Techniques 3. Compound Techniques.

The Passive techniques do not require any direct input of external power; rather they use it from the system itself which ultimately leads to an increase in fluid pressure drop.

The active techniques external power is used to facilitate the desired flow modification and the concomitant improvement in the rate of heat transfer. When any two or more of these techniques are employed simultaneously to obtain enhancement Called as Compound technique.

II. EXPERIMENTAL SET UP

The experimental study on passive heat transfer augmentation using square jagged twisted tape inserts for no.

of twists with a copper as a material were carried on in a single phase flow heat exchanger having the specifications as listed below:-

Fig: Experimental set up for forced convection

Fig: Copper square jagged Twisted tape (y=5.2)

Fig: Copper square jagged Twisted tape (y=4.2)

Fig: Copper square jagged Twisted tape (y=3.2)

Experimental set up consists of test section having tube in tube heat exchanger. Inside tube is of Copper and outside tube is of Stainless Steel. Four thermocouples are connected to the test section, two at the inlet and two at the outlet of hot and cold water respectively. Two rotameters are connected at inlet of cold and hot water to measure the flow rates. Also control valves and bypass valves are provided at inlet of both the rotameters. Two centrifugal pumps are used to circulate the cold and hot water. Two tanks are used for storing the hot water and cold water. Electric heater is attached to the hot water tank having capacity of 1500watt.

To measure the pressure difference between inlet and outlet of test section of hot fluid inverted U-tube manometer is used. Different twisted tape inserts are used. The current meter is used to measure the velocity of flow through inserted tube.

III. SPECIFICATIONS OF INSERTS

Material: Copper

i. Width of twisted tape = 12mm ii. Twist ratio = 5.2, 4.2, 3.2.

iii. Length of insert = 100cm iv. Thickness of inserts = 3mm Sample calculations

Water flows through copper tube .The cold water flows through annular space the flows of fluids are in opposite direction The it is counter flow arrangement The heat transfers from hot water to cold water we will calculate heat given by hot water, heat absorbed by cold water and average heat transfer as follows :

1) Properties of Water

a) Properties of hot water – calculated at mean bulk temperature

T_bh =

2 T + Th 1 h 2

b) Properties of cold water T_bc =

2 T + T_{c1 c2}

2) Heat given by hot water

Q_h = m_h





Q_c = m_c





© 2015, IERJ All Rights Reserved Page 3

Q_avg. =

2

C

h Q

Q 

5) Overall heat transfer coefficient Q_avg. = UAsTm

a) Surface area of tube

A_s = diL

b) Logarithmic mean temperature difference

T_m =

 

 

ln

2 1

T T

T T











T1 = Th1 – Tc2

T₂ = T_h2 – T_c1

6) Nusselt Number of cold water flowing through the annular space.

Nu_o = 0.023(R_eo)^0.8 (P_r)^0.3

Reo =



UoDh

D_h = D_i - d_o

Continuity equation – m_c = A_oU_o

7) Heat transfer coefficient of cold water flowing through the annular space.

Nu_o =

K

h oD

h hence, h_o =

h o

D K Nu 8) Heat transfer coefficient of hot water flowing through the tube.

o

i h

h U

1 1

1   hence, hi =





1 1

1



9) Experimental Nusselt Number of hot water flowing through the tube.

Nu_i = K

i id h

10) Theoretical Nusselt Number of hot water flowing through the tube (Dittus Boelter equation).

Nu_i = 0.023(R_ei)^0.8 (P_r)^0.3 11) Experimental Friction Factor

a) _P_gh

b)

i i

d U P fL

2

 2





2

2

i i

LU h f  gd

12) Theoretical Friction Factor























 

 











33 . 6 0

Re 50 10 1 0055 . 0

i

f

The procedure for calculating the various parameters for with and without inserts is given below. The experimentation was done at constant heat supply; hence the calculations are done at constant heat supply. A sample observation table is shown below to understand the parameters need to be observed during experimentation.

Results

Following Figures shows the relation of Nusselt number and Reynolds number for plain copper Square

jagged twisted tapes with twist ratio 5.2, 4.2 and 3.2 respectively. In the increasing range of Reynolds number, there was a continuous improvement in Nusselt number with the use of plain twisted tapes. Nusselt number increased with increase in Reynolds number and decrease in twist ratio. At maximum Reynolds number of 16000 and for smallest twist ratio of 3.2, the Nusselt number was 2.5 times that of plain tube without insert.

Fig: Nu v/s Re for plain tube and sq. jagged twisted tape. (Y

= 5.2)

Fig:Nu v/s Re for plain tube and sq. jagged twisted tape. (Y

= 4.2)

Fig:Nu v/s Re for plain tube and sq. jagged twisted tape. (Y

= 3.2)

© 2015, IERJ All Rights Reserved Page 4 Fig: f v/s Re for plain tube and plain twisted tape with

square jagged TT (Y = 5.2)

Fig: f v/s Re for plain tube and plain twisted tape with square jagged TT (Y = 4.2)

Fig: f v/s Re for plain tube and plain twisted tape with square jagged TT (Y = 3.2)

IV.

C

Experimental investigation of heat transfer and friction factor characteristics of circular tube fitted with full-length square jagged twisted tape inserts of different twist ratio has been presented. Calculations and curve fitting is done and correlations are obtained. The results showed that there was an appreciable enhancement in heat transfer and that the heat transfer is more with lesser twist ratio.

I. Maximum increase in Nu for square jagged copper twisted tape with y = 5.2 was found to be 44 % . II. Maximum increase in Nu for square jagged copper

twisted tape with y = 4.2 was found to be 82 % III. Maximum increase in Nu for square jagged copper

twisted tape with y = 3.2 was found to be 154%

IV. Friction factor is increased by 12 % with y=5.2 in Square jagged copper twisted tape.

V. Friction factor is increased by 27 % with y=4.2 in Square jagged copper twisted tape

VI. Friction factor is increased by 51 % with y=3.2 in Square jagged copper twisted tape

VII. We observe that with an increase in the Reynolds number (Re) ranging from 5000 to 16000, the heat transfer coefficients increases for square jagged copper twisted tape inserts by 44% to 154% with respect to plain tube whereas the friction factor decreases with increase in Reynolds number.

VIII. The square jagged copper insert with twist ratio 3,2 shows higher Nusselt number with 154%

increase in enhancement and friction factor minimum of 51% as compared to the plain tube.

IX. When experimental values are compared for Nusselt number then the square jagged twisted tape insert of twist ratio3.2 and 12 mm width has highest value of 2.54 and when friction factor values are compared then it is having 1.51as compared to the plain tube, hence proving the insert to be better in terms of heat transfer enhancement and also at lesser pumping power.

X. The increase in pressure drop by increasing number of twist ratio is not as significant as the increase in Nusselt number. The optimum value obtained for minimum pressure drop is for square jagged copper twisted tape with twist ratio 3.2

XI. MATLAB program is a reliable way for the prediction of results due to its high accuracy and flexibility that can be used to represent the experiments precisely

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