Heat Transfer By Free And Forced Convection LONG Report Ismail PDF

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ME-406 Mechanical Lab III Summer 2016 Heat Transfer by Free and Forced Convection Long report

By: Ismail Diab Group 3 Darko Gjoreski Ismail Diab Yusuf Diab Peshwa Shukla Date of Experiment : July 17, 2016 Date of Submission : July 25, 2016 THE EDGE OF KNOWLEDGE

Table of Contents Abstract............................................................................................................................................3 Introduction......................................................................................................................................4 Experimental Methodology.............................................................................................................6 Theoretical Principals......................................................................................................................7 Procedure.......................................................................................................................................12 Sample Calculations......................................................................................................................13 Results and Discussion..................................................................................................................20 Conclusion.....................................................................................................................................23 Reference.......................................................................................................................................24 Appendix........................................................................................................................................25

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Abstract The objective of the heat transfer by free and forced radiation experiment was to experimentally determine the overall coefficient of heat transfer for the flow of heat through a condenser tube wall, in free and forced convection, and comparing it to the coefficient of heat transfer obtained from empirical correlations, lastly to estimate the heat loss from the equipment from combined convection and radiation. The heat inside the condenser tube wall depends on whether or not the free or forced convection is developed in the condenser. Capturing data for the temperature difference across the tube wall for natural and forced convection, the overall heat transfer coefficient was determined.

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Introduction Through the study of thermodynamics it is learned that energy can be transferred by interactions, which are work and heat, of a system with its surroundings. And so the heat transfer is the “thermal energy in transit due to a spatial temperature difference”.

Figure 1 Conduction, Convection, and Radiation Heat Transfer Modes

Three types, modes, of head transfer processes that have been studied over the year are conduction, convection, and radiation (as shown above in Figure 1). When there is a heat transfer occurring between a surface and a moving fluid, where the temperatures are different, it is referred to as convection. Thermal Radiation is referred to where all surfaces of finite temperature emit energy in the form of electromagnetic waves. In the absence of an intervening medium, there is a net heat transfer by radiation between two surfaces at different temperatures. Lastly, conduction is referred to as when a temperature gradient exists in a stationary medium where heat transfer occurs across the medium. In further discussing about convection, as the main objective of this experiment is to determine the overall coefficient of heat transfer for the flow of heat through a condenser tube wall in both free and forced convection, let’s begin free or most notably natural convection. Free

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convection “originate when a body force acts on a fluid in which there are density gradients. The net effect is a buoyancy force, which induces free convection currents. In the most common case, the density gradient is due to a temperature gradient, and the body force is due to the gravitational field.is the process. Also Since free convection flow velocities are generally much smaller than those associated with forced convection, the corresponding convection transfer rates are also smaller” (Bergman et al 562). Continuing, in forced convection, which can have external/ internal configurations, the fluid is forced to flow over the surface or in a tube (ex. a pump or fan). For external flow “the boundary layer development on a surface is allowed to continue without external constraints. In contrast, an internal flow, such as flow in a pipe, is one for which the fluid is confined by a surface. Hence the boundary layer is unable to develop without eventually being constrained. The internal flow configuration represents a convenient geometry for heating and cooling fluids used in chemical processing, environmental control, and energy conversion technologies” ((Bergman et al 490).

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Experimental Methodology In heat transfer by free and forced convection experiment the equipment used in conjunction to obtain the desired data information consisted of an electric boiler, a condensing tower, a closed jacket and single aluminum tube, a boiler supply tank, and a weight tank and thermocouples. The electric boiler produces the steam that is fed into a condensing tower. Within the condensing tower is a closed jacket and a central single aluminum tube. The cooling water, which is provided by a head control system and further helps in performing the experiment with free and forced convection, moves upward through the inside of the condenser tube, which causes the steam to condense on the outside surface. Steam also condenses on the inside surface of the jacket as heat escapes out into the room. In order to provide and maintain a constant level in the boiler a boiler supply tank is used, thus ensuring that the mass within the system remains constant throughout the experiment. The condensation resulting from the tube wall and shell wall are collected separately by means of drain tubes, the cooling water flow moving through the tube is collected in a weight tank that is mounted on a scale. Lastly the thermocouples, all copperconstantan (type t), used to measure the temperatures are monitored using a high impedance millivolt meter.

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Theoretical Principals In performing this heat transfer experiment our focus lied on the transfer of heat by means of free and forced convection in condensation. Heat exchangers are devices whose primary responsibility is to transfer heat from one fluid/gas to another fluid/gas without the two fluids/gases mixing to coming in contact with each other. The application of heat exchangers range from heating to cooling systems. In our experiment we dealt with a condenser which is a heat exchanger where “a vapor is cooled to the extent that condensation takes place, heat is transferred in a fundamentally different manner than when heat is added (or taken away) from a fluid without a change of phase. The condensation of vapor liberates a considerable amount of energy, but at the same time a barrier is set up in the form of a liquid film which either partially or completely covers the cooler surface. The thickness of this film is influenced both by its viscosity and by the position and height of the surface (its geometry). Drainage from a vertical or an inclined surface is naturally more rapid than from a horizontal one and the film is accordingly thinner; but if the vertical height is great, the accumulation of condensation at the lower portion of the surface will thicken the film and make the lower portion less effective than the upper in transmitting heat” (Heat Transfer by Free And Forced Convection). Furthermore, in free convection the fluid motion is caused by means such as gravity, buoyancy, rise and fall of warmer can cooler fluid respectively, etc., and in forced convection the flow of fluid is forced over a surface or in a tube through internal/external means, fans, pumps, etc. Below will follow the theoretical aspects of both forced and free convection; Forced Convection:  Equation 1  Equation 2 where, DIAB 7

Thermal energy transfer per unit time and surface area h= coefficient of convection Surface Temperature = Ambient Temperature

Thermal energy transfer per unit time A= cross are section

The coefficient of convection, h, has a high dependence on the fluid properties, the roughness of the surface of the solid, and the type of fluid flow, which can be laminar or turbulent. Figure 2 shows an illustration of this topic,

Figure 2: Forced Convection

Figure 3 below illustrates the phases of fluid flow over a surface, where the velocity and the temperature of the fluid approaching the plate is uniform at U infinity and T infinity.

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Figure 3 Velocity Boundary Layer (Forced Convection)

The Nusselt number represents the improvement of heat transfer through a fluid as a result of convection relative to conduction across the same flu id layer as shown in Equation 3.

 Equation 3 where, δ is the characteristic length  Equation 4 where, = Density V= Velocity = Viscosity

 Equation 5 where, ν=kinematic viscosity, µ / ρ (m2 /s) c α= thermal diffusivity, k/(ρ ∙ Cp) (m2 /s) µ= dynamic viscosity, (N ∙ s/m2 ) Cp= specific heat capacity, (J/kg ∙ K) ρ = mass density ,(kg/m3 ) k=thermal conductivity, (W/m∙ K)

Forced Convection of Water Flowing Inside:

 Equation 6 where,

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Coefficient of forced convection c= constant which acceptable value of 0.023 = inner diameter of inner tube

Velocity: Inside of the inner tube

 Equation 7 where, = rate of change of mass (kg/s)

Free Convection: Due to the low fluid velocity associated with natural convection the heat transfer coefficient encountered in natural convection is just as low. Figure 4 below is an illustration of free convection where an object is exposed to outside high temperature whose heat is transferred into the object and then consequently from the object to the surrounding environment.

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Air

Air

Figure 4 Free Convection

Natural Convection of Water Flowing Inside:

 Equation 8 where, = Coefficient of Free or Natural Convection C= constant, Table 19-3 L= length of cylindrical column = volumetric coefficient of expansion of water = Temperatures average, C

Film Convection Coefficient Outside:

 Equation 9 where, =film coefficient outside c= constant 1.66 for vertical surface and 1.282 for a horizontal surface D= diameter of a vertical tube, m = latent heat = condensing temperature minus surface temperature, C

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Heat Transfer Free Convection:  Equation 10

 Equation 11

 Equation 12

 Equation 13

Procedure The first phase of the experiment began with the heat transfer by free convection, where valve (B) was slowly adjusted until the water level in the condenser water level control tank was at the free convection level, the level had to be maintained throughout for the first phase of the experiment. The boiler supply tank was filled with water to the fill mark, keeping in mind to not allow the water in this tank to go below the low-level mark. Petcock (A) was opened, which allowed the water to flow from the boiler supply tank into the boiler, also keeping in mind to only fill it to the fill mark, and then it was closed. The thermocouples were permanently connected to a multiple position switch that was mounted on the table and so a millivoltmeter

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was connected to the switch. An ice point reference junction had to be provided for. The power to the electric boiler was turned on and set to the variac so that the ampmeter read 6 amps. When the steam temperature entering the condenser (t 1) reached between 220 and 240 degrees in Fahrenheit the boiler was carefully refilled by adjusting the petcock (A) and thereafter in order to maintain the water level at the red mark. The input of 6 amps to boiler was also maintained, never allowing the boiler to run dry while connected to the power supply. The system was given time to reach equilibrium which was managed by keeping an eye on all the temperatures until they stabilized, this took about 15 minutes. The outlet containers were put in place for the cooling water and tube and shell condensation. The amount collected in each respective outlet container was recorded along with the time duration of collection, all data was recorded every 5 minutes for a period of 20 minutes. Moving on to the second phase of the experiment heat was transferred by means of forced convection, the steps previously mentioned for phase one of the experiment was repeated after adjusting and maintaining the water level in the condenser water level control tank at the level marked “Forced”.

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Sample Calculations Given Dimensions: Di = 0.01905 m, ri = 0.00953 m Do = 0.0206 m, ro = 0.01032 m L = 0.635 m, Surface Area = 0.0386 m2, Cross Sectional Areal = 2.85 x 10-4 m2 Forced Convection:

Using the mean temperature the other properties of the hforced is obtained and shown below:

Having all the variables defined the solution for hforced can be identified.

Natural Convection:

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Using the mean temperature the other properties of the hnatural is obtained and shown below:

The value of the constants C and n will be defined based on what type of flow exists. If the flow is laminar, C = 0.45 and n = 1/4, but if the flow is turbulent then C = 0.13 and n = 1/3.

Having all the variables defined the solution for hnatural can be identified.

Using the mean temperature the other properties of the ho, steam is obtained and shown below:

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247

Using the mean temperature the other properties of theho, steam is obtained and shown below:

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Empirical Calculation for Natural Convection:

Empirical Calculation for Forced Convection:

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Results and Discussion Natural Convection

1326.28

63.31

65.11

165.02

792.41

83.56

1359.90

63.29

65.20

166.03

782.90

85.46

1376.54

64.03

66.1

170.72

752.51

89.73

1360.37

65.30

67.39

171.79

777.81

81.41

1382.44

65.56

67.45

175.1

741.27

92.92

Forced Convection

411.85

50.94

48.77

39

1467.8

12.48

409.88

50.76

48.44

40.74

1406.91

12.99

406.20

49.50

79.34

63.79

1461.76

15.63

330.16

47.49

44.84

32.59

1575.03

10.18

402.03

38.06

37.30

27.48

1561.01

8.59

After completing the sample calculations for one set of data points for both free and forced convection, the Microsoft Excel software was used to make all the calculations for the entire set of data point. Furthermore, what was done was completed the computation sheet provided and the value of U0 was determined, the overall heat transfer coefficient through the condenser tube wall based on the tube outside surface area. This value was then compared with

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that obtained from empirical correlations available in the literature. Lastly, the rate of heat loss from the equipment was determined. As seen in equation 28, equation 30, and equation 31 the experimental Coefficient of heat transfer was computed (for forced, natural, and steam respectively). Constant such as density, viscosity, and specific heat were obtained through the use of the average temperature of the heat water. Given the experimental coefficient of heat transfer for the condensing steam, the steam behavior in the heat exchanger is not ideal, however, it can be assumed to be natural convection. The heat loss, as calculated in equation 41 and equation 44 for the natural and forced convection. The values obtained were influenced by the conditions of the equipment, which consisted of it being old and leaks from the use in multiple times over the years. In order to better understand the experimental values compared to empirical it was important to estimate the amount of heat lost in the system, where the process consisted of the steam heating the water, the steam condensing at the outer surface of the cylinder. Concluding the experimental calculations was the calculation of the empirical values that served in order to compare it to the experimental results. The empirical results did not take into account the heat losses in the system, which led to the inconsistency within the empirical calculations as seen in Table 6 and Table 8. Lastly equations 34 and equation 38 provides the values of U0 for natural and forced convection.

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Conclusion In concluding the heat transfer by free and forced convection laboratory, we have spent more than an hour for both test, because while developing the forced convection part of this experiment we forgot to measure the initial weight of the cooling water tank. Discrepancies between the experimental and the empirical calculations were found, the experimental values are higher than the empirical values. Some errors from the experiment make it hard to analyze the data, but most of result arc coherent with the theory. The results could be improved if the experimental system and equipment were much newer and up to date, the current system does not provide very precise results and the students could make some mistakes reading the value.

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Reference "Convective Heat Transfer." Convective Heat Transfer. Web. 5 Dec. 2015. . "Forced Convection." Web. 5 Dec. 2015. . "Heat Transfer." Heat Transfer. Web. 5 Dec. 2015. . "Heat Exchangers." How Do Heat Exchangers Work? Web. 16 Dec. 2015. . “HEAT TRANSFER BY FREE AND FORCED CONVECTION” Lab Manual. Web. 5 Dec. 2015.. Incropera, Frank P., and David P. DeWitt. Introduction to Heat Transfer. 6th ed. New York: Wiley, 2011. Print. "Natural Convection." Web. 5 Dec. 2015. . "What Is a Condenser?" AC / Heat Pump Condensers Explained. Web. 5 Dec. 2015. .

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