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HEAT TRANSFER LAB...

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ME-406 Heat Transfer by Free and Forced Convection

Abstract

The purpose of this experiment is to analyze the operation of a heat transfer phase inside a condenser tube and obtain the values for the overall heat transfer coefficient as well as the amount of heat loss at two different forms of convection. The experiment will focus on the performance of a condenser tower tube (heat exchanger) to show how free convection and forced convection affect the amount of heat energy being transfer by delivering water and steam into the system. The values for convection heat loss as well as overall heat transfer coefficient will be calculated using temperature readings from thermocouples recorded on LabVIEW, as well as mass and volume measurements that are gathered manually. Steam and water will travel into the system causing both of its heat energy particles transferring to create condensation from the steam. The energy needed to generate steam will be provided by an electric boiler that is linked to a water tank. The values for the heat transferred to the condensed liquid were found to be different for forced and free convection; and the experimental values for heat loss and overall coefficient of heat transfer were within an acceptable error range when compared to empirical values.

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ME-406 Heat Transfer by Free and Forced Convection

Table of Contents

           

Abstract………………………………………………………………………….1 Introduction……………………………………………………………………...3 Experimental System…………………………………………………………....4 Procedure…………………………………………………………………….….7 Theory…………………………………………………………………………...8 Sample Calculations………………………………………………………….….10 Results…………………………………………………………………………...12 Discussion……………………………………………………………………….15 Conclusion………………………………………………………………………16 References………………………………………………………………………17 Appendix…………………………………………………………………….….18 Preliminary Report……………………………………………………………...21

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ME-406 Heat Transfer by Free and Forced Convection

Introduction

Convection is one type of heat transfer that occurs when particles with heat energy takes over particles with less heat energy in liquids and gases. This process of convection can happen naturally or by an external force. One mechanical device that shows process of convection is called a heat exchanger. A heat exchanger is a system that allows heat of a fluid to transfer to another fluid without the two fluids having to mix together or coming into direct contact. Figure 1-A, gives a visualization of what happen two different fluid travels inside a heat exchanger.

Figure 1-A: Heat Exchanger (behavior of fluids)

In this experiment, 2 fluids travel inside a condenser tube and exchange in heat transfer occurs. As cold water passes through the tube with hot steam, it makes the steam condense on the outside surface. Additionally, there is also condensation on the inside surface as there is another condensing path mixing with the room temperature. The objective is to determine the overall coefficient of heat transfer for the flow of heat through the condenser tube wall in both free and forced convection, compare it to that obtained from empirical correlations available in the literature, and to find heat loss from the equipment of combined convection and radiation.

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ME-406 Heat Transfer by Free and Forced Convection

Experimental System

Figure 2-A: Full System Sketch Drawing

Figure 2-B: Boiler and Condenser Tube (Steam) 4

ME-406 Heat Transfer by Free and Forced Convection

Figure 2-C: Cool Water supply (Cold water)

Figure 2-D: Condensation Tubes 5

ME-406 Heat Transfer by Free and Forced Convection

Figure 2-E: Condensed Water

Figure 2-F: Virtual Logger (Temperature recorded over time) 6

ME-406 Heat Transfer by Free and Forced Convection

Procedure

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ME-406 Heat Transfer by Free and Forced Convection

Theory While recording data and gathering the amount of water being condensed by the steam inside the condenser tower, convection was analyzed as the method of heat transfer. Steam was generated by the boiler and stored inside the condenser tube. Con denser tube has a cold fluid (Cool water) traveling inside which cause the steam to condense at a fast rate due to convection from hot energy (steam) and Cold water low hot energy. Also, the condenser tube had air room temperature travel inside which cause the steam to condense, but at lower than the cold water due to temperature differences. In order to prove or test this out we recorded the temperatures relative to time during the operation of the system. Two forms of convection were test out to analyze the overall heat transfer at those two difference states. Free convection is when the cold-water supply was being pushed into system by gravity force and no external force such a pump or turbine. For the forced convection we apply an external force that generated the cooling water to travel through the system at faster rate. Therefore, we can acknowledge how which state has the more condense in water, heat loss, and overall heat transfer coefficient. The Following formulas were used in order to calculate the overall heat transfer coefficient of the free and forced convection. Empirical formula, determines the heat transfer rates and the total coefficient of heat transfer for the condensed steam:

LMTD=

△ T 2−△ T 1 △T2 ln( ) △T1

C ,∈¿ H ,∈¿−T ¿ △ T 1=T ¿ △ T 2=T H , out −T C ,out

´ m=

m ( o )−m (i) t ( o )−t (i)

q=m∗△ ´ enthalpy Cold water U=

q Ai∗LMTD

Where, LMTD = Logarithmic mean temperature; △ T 1 = Change in temp. in inner tube; △ T 2 = Change in temp. in outer tube; m=¿ Mass flow rate of fluid; U = total heat transfer. ´ 8

ME-406 Heat Transfer by Free and Forced Convection

Heat Transfer Coefficient (Steam), the following formula is used: H =C

[

( K 3 ρ2 hfg ) ( Dμ T m , steam )

]

n

T ¿ H ,∈¿+ T H ,out ¿¿ ¿ ¿ T m ,steam =¿

Convection Coefficient (Water), the following formula is used: H w=

(N u∗K ) D

In order to find Nusselt number, the Reynolds number should be determined letting us know if the fluid behavior is laminar or turbulence. Re =

4m ´ Pi∗D ¿∗μ

Convective Coefficient formula, assuming surface temp. outer surface uniform and temp. of steam based on the cooling tube lower surface temp.: C∗K H air = D

[(

)(

C p∗μ D 3∗ρ3∗△ T∗g ∗ 2 K μ

[ Gr∗ Pr ] )] = C∗K D n

Value of C is based on Rayleigh No. g∗β∗( T s−T ∞ )∗D3 v∗ μ

Ra =

Overall Heat Transfer Coefficient formula is the following: 1

V o=

( h1 )+( K ) ln( D )+( h1 ) w

ri

Do

s

i

s

Heat Loss from the equipment is calculated as the following: Heat Loss=Q steam−Q water −Q conv

Therefore, 9

n

ME-406 Heat Transfer by Free and Forced Convection

Q steam=m ´ ∗hfg Q conv =hconv∗A o∗△ T Q water =m∗△ ´ enthalpy Cold water

Sample Calculations Empirical formulas LMTD=

△ T 2−△ T 1 ln

( ) △T2 △T1

=

20.886−91.519 =47.60 Celcius 20.886 ln 91.519

(

)

3.75 lb ∗.459 kg 5 min ∗1 min m( o )−m (i) 1 lb m= ´ =.00573 kg /s = 60 sec t ( o )−t (i)

q=m∗△ ´ enthalpy Cold water =.00573 U=

(

)

kg KJ 100.68 KJ ∗ 343.35 =1.39 kW − s Kg Kg

q 1.39∗103 W =762.94 w /m2 k = Ai∗LMTD (.038 m 2∗47.60)

Heat Transfer Coefficient Steam

H =C

[

( K 3 ρ2 hfg ) ( Dμ T m ,steam )

]

n

= ( .94 )

[

(

)(

)

2

3 9.57∗102 kg w 3 ∗ ∗( 2233.21∗10 ) .681 3 mk m

( .0233 m )∗

(

−4

)

2.75∗10 kg ∗( 108.88 ) 2 m s

]

.25

=5186.2w /m 2 k

Convection Coefficient Cold Water kg ) s 4m ´ = Re = =1,686.75 Pi∗D ¿∗μ 3.14∗.0083 m∗(5.218∗10−4 kg /m2 s ) 4∗(.005737

Nusselt Number, Laminar Flow: 4.364 (N ∗K ) = H w= u D

(4.364)∗(.645 .0083 m 10

w ) mk

=339.13 w/m2 k

ME-406 Heat Transfer by Free and Forced Convection

Convective Coefficient m 9.81 ∗( 3.02∗10 k ) ∗( 79.624 )∗( .0233 m ) ( s ) =8.018∗10 = m kg (1.87∗10 s )∗(1.99∗10 m s ) −3

Ra=

g∗β∗( T s−T ∞ ) ∗D3 v∗μ

−1

3

2

4

2

−5

−5

2

C = .48; n = .25

H air =

C∗K D

[(

)(

C ∗μ D 3∗ρ3∗△ T∗g ∗ p 2 K μ

)]

n

(

= ( .48 )∗

[

(

)

(

kg m ∗( 79.62)∗ 9.81 2 3 m s 2

( 1.99∗10−5 )

.0283 ∗ .0233

Overall Heat Transfer Coefficient

(

)

3

( .0233 m ) ∗ 1.066

.0283

)

.00405 ln .0095 +( 1 ) 16 5186.2 .0083 ¿ 1 +¿ 339.13 1 1 = V o= ¿ ri Do 1 1 +( ) ln + Ks Di hs hw

(

)

( )( ) ( )

Heat Loss of Heat Exchanger (Condenser Tube)

(

Q steam=m∗h ´ fg= .650∗10−3

(

Q conv =hconv∗A o∗△ T = 2.76

)(

)

kg J 3 ∗ 2,233.21∗10 3 =1.452∗10 W s kg

)

w ∗( 81.68−24.31 )∗( .0082∗3.14∗.66 )=26.6 W m2 k

Q water =m∗△ ´ enthalpy Cold water =1.39 kW 3 3 Heat Loss=Q steam −Q water −Q conv=( 1.452∗10 W )− ( 1.39∗10 W )−( 26.6 W )=35.6 W

11

w mk

)∗1∗(1

ME-406 Heat Transfer by Free and Forced Convection

Results Free Convection

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ME-406 Heat Transfer by Free and Forced Convection

Forced Convection

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ME-406 Heat Transfer by Free and Forced Convection

Constant Values

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ME-406 Heat Transfer by Free and Forced Convection

Discussion

During the operation of the experiment in free convection the temperature measurements were generally higher than the ones measured during the forced convection analysis. This observation can be explained by the fact that the water circulating through the condenser tube was in contact with the steam for a longer period of time, on the other hand, in forced convection phase the fluid moved faster causing less of its heat energy particles to transfer with the steam energy particles. The mass flow rate of the condensation fluid exiting from the condenser tube was found with the use of the volume of water collected in a beaker and then dividing such volume by 5 minutes. This was done for both phases of convection and it was quickly realized that we received more condensed liquid through the forced convection. This is due to the change in temperature for both fluids, water and steam. As the external force pushed the water at faster rate helping it stayed at low temperatures, the steam will have to condensate faster because of the other fluids low temperature.

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ME-406 Heat Transfer by Free and Forced Convection

From the empirical formulas we were able to analyze any anomalies in our calculations. In the result tables, we can acknowledge a difference in the overall heat transfer coefficients in both free and forced convection comparing to the empirical results. Other factors that can also apply due to error analysis in conducting the experiment, was that we had to replace the beaker as it was getting full which caused us to have small loses in volume of the condensed water. Also, we constantly had to feed the tank with water to maintain it at certain level for water to travel with ease inside the system.

Conclusion In conclusion, the values for heat transfer rates as well as the overall heat transfer coefficient were found with the use of different calculations specified in previous sections. The empirically calculated results varied as compared to the experimental results. This can be attributed to the downfalls the experiment had that could have poorly affected our results and therefore, our calculations. The measurement system of filling up beakers seemed fairly unreliable. Since the trials were 25minutes long often times the beakers would become full and require emptying, in which water was not being caught and mass was lost. In addition, there was pressure accumulating inside of the cylinder itself that was not accounted for. Free convection showed to have higher working temperatures than forced convection after completing both analyses. The experiment clearly demonstrated the functionality of tube condenser and how it allows the heat transfer between fluids without mixing.

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ME-406 Heat Transfer by Free and Forced Convection

References Book: Holman, J. P. (2012). Experimental Methods for Engineers. Boston: McGraw-Hill/Connect Learn Succeed. Lab Manual: https://njit2.mrooms.net/pluginfile.php/1217553/mod_resource/content/0/ME_406_LAB_MAN UAL_IC_ENGINES.pdf Property Table: http://homepages.wmich.edu/~cho/ME432/Appendix1Udated_metric.pdf Heat Exchanger Description: https://www.explainthatstuff.com/how-heat-exchangers-work.html 17

ME-406 Heat Transfer by Free and Forced Convection

Appendix Data Sheet:

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ME-406 Heat Transfer by Free and Forced Convection

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ME-406 Heat Transfer by Free and Forced Convection

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ME-406 Heat Transfer by Free and Forced Convection

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ME-406 Heat Transfer by Free and Forced Convection

Preliminary Report

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ME-406 Heat Transfer by Free and Forced Convection

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ME-406 Heat Transfer by Free and Forced Convection

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ME-406 Heat Transfer by Free and Forced Convection

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ME-406 Heat Transfer by Free and Forced Convection

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ME-406 Heat Transfer by Free and Forced Convection

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ME-406 Heat Transfer by Free and Forced Convection

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ME-406 Heat Transfer by Free and Forced Convection

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