Experimental Study of The Effect of Addition of Calming Section on The Heat Transfer Process In A Uniformly Heated Inclined Tube

. Experimental research has been conducted for combined convection to study the local and average heat transfer for hydrodynamically fully developed (the value of L/D of calming section was 22.2) and thermally developing laminar air flow in a uniformly heated inclined tube. The study covered a wide range of Richardson number (Ri) from 0.08 to 2.5, Reynolds number from (518 to 2041), and Rayleigh number from (2.4 × 10 5 to 3.9 × 10 5 ), with different angles of inclination { α = 0 0 (horizontal), ± 30 0 , ± 60 0 , ± 90 0 (vertical)} where the minus sign refers to opposing flow and plus sign refers to aiding flow. An empirical correlations were made for the average Nusselt number as a function of Rayleigh number and Reynolds number for all angles of tube inclination and compared with another correlations from previous works. Results show that the heat transfer process for low Richardson number is better than that for high Richardson number and the values of average Nusselt number ( Nu ) increase as Peclet number (Pe) increases for the same Rayleigh number.


Introduction.
Thermal buoyancy forces play a significant role in forced convection heat transfer when the flow velocity is relatively small and the temperature difference between the surface and the free stream is relatively large.The buoyancy force modifies the flow and the temperature fields and hence the heat transfer rate from the surface.Problems of heat transfer in tubes by combined free and forced convection have been the subject of investigations for many years.Mixed convection occurs in many heat transfer devices, such as nuclear power technology, some aspects of electronics cooling, supercritical boilers, tube heat exchangers, etc.The relative direction between the buoyancy force and the externally forced flow is important.In the case where the fluid is externally forced to flow in the same direction as the buoyancy force, the mode of heat transfer is termed assisting combined forced and natural convection.In the case where the fluid is externally forced to flow in the opposite direction to the buoyancy force, the mode of heat transfer is termed opposing combined forced and natural convection or mixed convection which is characterized by the dimensionless parameter representing the ratio of buoyancy and inertial forces.This parameter is called Richardson number, simply defined as: Various researchers investigated the effect of mixed convective flows in tubes using experimental methods.

Grassi & Testi [1],
Studied the regime of transitional mixed convection in a uniformly heated horizontal cylindrical pipe.Heat transfer coefficients were measured at 6 cross sections along the heated length, with 60 azimuthal positions being monitored at each section.A single Nusselt number correlation was proposed for the entire heat transfer, best fitting the 5760 experimental points with only 7 parameters.The maximum deviation from the measurements is 17.5%, with a 3% average.Yousef and Tarasuk [2], studied the influence of the free convection on forced air flow in the entrance region of an isothermal tube.The L/D ratio was varied from (6 up to 46), Reynolds number ranged from (120 to 1200), and the Grashof number ranged from (0.8×10 4 to 8.7×10 4 ).The average Nusselt number based on the log-mean temperature difference, ranged from (2 to 25.9), has been obtained.The heat transfer data were correlated according to the influence of the free convection, which was found to have a significant effect at points close to the tube entrance.Szetela and Sobel [3], used a Kerosene-type gas turbine fuel flow through a directresistance heated tube at uniform heat flux.The data presented were obtained at an entrance Reynolds number of approximately 1500 and led to two peaks in a tube wall temperature near the tube entrance.  .The results demonstrate an increase in the Nusselt number values along the cylinder as the heat flux increases and as the cylinder moves from the vertical to the horizontal position.
This study has been presented the effect of calming section which was used to produce hydrodynamically fully developed flow on the heat transfer process and discussed the direction effect of primary flow relative to the secondary motion generated by buoyancy effects and compared the results with previous works to knew the effect of shape of velocity profile (uniform or fully developed) at the entrance region of the test section and the value of L/D for test section on the heat transfer process.

Experimental Apparatus.
The schematic diagram of all apparatus and experimental rig is shown in Fig. (1-a).The heat transfer test section is a circular tube made from aluminum with dimensions (inner PDF created with pdfFactory Pro trial version www.pdffactory.com

Experimental Study Of The Effect Of Addition Of Calming Section On The Heat Transfer Process In
A Uniformly Heated Inclined Tube diameter 52.3 mm, outer diameter 59.3 mm, and length 1.2 m).The test section was thermally insulated by using asbestos rope and fiberglass with total thickness of about 65.3 mm to minimize heat losses to the surrounding as shown in Fig. (1-b).A uniform heat flux as a boundary condition was maintained by using an electrical heater wire (nickel-chrome) wound uniformly around the tube as a coil with 10 mm pitch and electrically isolated by ceramic beads to prevent any electrical contact between the test section and electrical heater as shown in Fig. (1-b).An eighteen asbestos sheath thermocouple (type K) were placed along the tube wall to measure the tube surface temperature.
To ensure that the air flows at the entrance of the test section hydrodynamically fully developed, an aluminum tube (calming section) which has the same dimensions of the test section was used for this purpose.The calming section was connected with the test section by teflon connection piece bored with the same inside diameter of the test section and calming section.The teflon material was chosen because of its low thermal conductivity so that the losses from the end of the test section can be reduced.

Experimental Procedure
Firstly, the inclination angle of the tube is adjusted as required.Then, the centrifugal fan is switched on to push the air through the open loop while the fan control valve was used for adjusting the required mass flow rate.
Then, the electrical heater switched on while the electrical input power adjusted by AC power variac to give the required heat flux.The apparatus is left at least two hours to establish steady state condition, during this time, the thermocouples readings is measured every half an hour by means of the digital electronic thermometer until the reading became constant, then, this reading is recorded as a final reading.During each test run, (angle of inclination of the tube, reading of the manometer, readings of the thermocouples, heater current, and heater voltage are recorded).

Data reduction
The total input power supplied to the tube can be calculated: The convection and radiation heat transferred from the tube is : Nu can be calculated as follows:

Results and Discussion A) Temperature and Local Nusselt number Variations
A total of 140 test runs have been conducted to cover seven tube inclination angles of 0 0 , 30 0 , 60 0 , 90 0 , -30 0 , -60 0 , -90 0 .The range of Rayleigh number from (2.4×10 5 to 3.9×10 5 ) and Reynolds number is varied from (518 to 2041).In this research, the effects of more important parameters on the heat transfer process were taken into consideration such as Ri, Pe, Ra, Re, α, that few figures has been covered.
The surface temperature variation for selected runs is plotted in Fig. 2 and Fig. 3 for various angles of inclinations and constant Richardson number Ri=1.2 & 0.08 (i.e., constant Reynolds number and heat flux which gives Grashof number); respectively.If the behavior of temperature distribution is studied alone, it has been observed that at the entrance of tube the surface temperature increases from a certain value along tube axis because the thickness of the boundary layer is zero.Then it gradually increases to maximum value when the boundary layer fills the tube.This region is called tube thermal entrance in which the heat transfer results gradually decreases.Beyond this region the surface temperature is decreased due to the laminarization effect in the near wall region (buoyancy effect) and due to tube end losses.It is clear from these two figures that the values of temperature increase as the inclination angles deviates from horizontal to vertical position for both aiding and opposing case.This behavior could be elucidated and attributed to that when heat is transferred through the tube wall at horizontal position (i.e., the primary flow is perpendicular to secondary flow), the density of the air near the wall decreases.Then, the body forces acting due to this density change cause the hotter air to climb upward along the side walls of the tube while the fluid of higher density (lower temperature) moves downward to the tube core then to the lower portion of the tube wall.This related to increase the secondary flow pattern and it has been expected two identical PDF created with pdfFactory Pro trial version www.pdffactory.com

Experimental Study Of The Effect Of Addition Of Calming Section On The Heat Transfer Process In
A Uniformly Heated Inclined Tube longitudinal vortices have generated which superimposed on the main forced flow as has been observed by lavine et al [14].These vortices will be weaken as the angle of inclination deviated from horizontal to vertical position in which the vortex diminishes because the primary and secondary flows are parallel with the same or opposite direction.It is necessary to mention from these two figures that the values of temperature for opposing flow are higher than that for aiding flow at the same axial distance and vice versa for the values of local Nusselt number as shown in Fig. 4 and Fig. 5 which show the variation of local Nusselt number with dimensionless axial distance (inverse Greatz number) for (Ri=0.08 and 1.2) respectively, and for various angles of inclination.This behavior can be attributed to that when the flow is opposing, the direction of main flow opposes the buoyancy force which tends to retard the fluid near the heated wall (the velocities decrease in this region) leading to decrease the natural convection.While in the case of aiding flow, the buoyancy force and the main flow are in the same direction.So, the velocities near the heated wall increase and the free convection assists the main flow in removing heat from the tube surface and improving the heat transfer process.The dotted curve in the last two figures represents the theoretical pure forced convection (TPFC) based on constant property analysis of shah and London [13].It can be seen also that the values of the local Nusselt number for all angles of inclination are higher than that predicted for (TPFC) for low values of Richardson number as shown in Fig. 4, and these values begin to decrease as Richardson number increases as shown in Fig. 5 at the same (Gz -1 ).The observations of these figures can be regarded as conclusions listed later in this item.

B) Average Nusselt number Variations
Variations of the Nusselt number with the increase of the Peclet number at selected Rayleigh number are presented in Fig. 6 and Fig. 7 for horizontal and vertical position; respectively.It is evidenced that at lower Peclet numbers the effect of Rayleigh number is significant.When the Peclet number is larger than certain value (Pe > 1450) the heat flux data points close to each other for different Rayleigh numbers (Ra<3.3×10 5 ) which means that the average Nusselt number becomes independent of Ra and forced convection prevails.This observation can also be confirmed by Figs. 8 &  The practical importance of these empirical correlations presented in this study will be used for designing of some heat transfer process.It is evident from these figures that the behavior of heat transfer process for the present work agrees well with the work of Jadoa [12] in which the average Nusselt number values decreases as the average value of Ra/Re increases with a higher values for the present work.In the other side, this behavior is reversal in the work of Hussein et al [6-9].To discuss this difference, a comparison among these three works should be made to know the difference and similarity among them as follows in the table (table B): As be shown in table (B), the extension in the range of values of Richardson number for Hussein work it compared with the present work leading to reverse the behavior of heat transfer process.In another words, the heat transfer process improves as the average values of Ra/Re increases (i.e., the natural convection is the dominant factor in the heat transfer process) for the high range of Richardson number and as Ra/Re decreases for the low range of Richardson number such as the present work (i.e., the forced convection is the dominant factor in the heat transfer process).

Conclusions
Based on the experimental results presented here in, the following conclusions were made: 1.The horizontal position gives lower values of temperature and higher values of local Nusselt number than other angles of inclination.2. Generally, the heat transfer process in the case of aiding flow is better than that in the case of opposing flow at the same angle of inclination and Richardson number.

Experimental Study Of The Effect Of Addition Of Calming Section On The Heat Transfer Process In
A Uniformly Heated Inclined Tube than that predicted for (TPFC) at the same local location which is represented by inverse Greatz number (Gz -1 ). 4. For Ri=1.2 (i.e., strong mixing process), it can be concluded that a.The values of Nux at α=±60 0 and =±90 0 (vertical), are lower than that predicted for (TPFC), i.e., these angles of inclination give low values of heat transfer coefficient inspite of higher value of Richardson number.b.The value of Nux at α=30 0 at the end tip point of thermal entrance length approaches from the asymptotic value of (TPFC), i.e., the point at which the value of Nux becomes constant.c.The values of Nux upstream at α=-30 0 are similar to that predicted for (TPFC), then decrease downstream to be lower than that in (TPFC).d.The local Nusselt number values for α=0 0 (horizontal position) and α=30 0 are higher than that predicted for (TPFC) at the same local location (Gz - ).

At lower
hence the radiation heat flux is very small and can be neglected So qcr ≈ q = convection heat flux …(5) The local heat transfer coefficient can be obtained as:PDF created with pdfFactory Pro trial version www.pdffactory.comEng.& Tech.Journal, Vol.28, No.15, 2010 Experimental Study Of The Effect Of Addition Of Calming Section On The Heat Transfer Process In A Uniformly Heated Inclined Tube x the air properties are evaluated at the mean film air temperature [10].Local mean film air temperature.The local Nusselt number (Nu z ) then can be determined as: conductivity of air = 0.6099 W/m 2 °C The average values of Nusselt number

3 .
For Ri=0.08 (very lower than one i.e., mixed convection with dominated forced convection), the values of local Nusselt number for all angles of inclination are very higher PDF created with pdfFactory Pro trial version www.pdffactory.comEng.& Tech.Journal, Vol.28, No.15, 2010

& Tech. Journal, Vol.28, No.15, 2010 Experimental Study Of The Effect Of Addition Of Calming Section On The Heat Transfer Process In A Uniformly Heated Inclined Tube
Results show that at PDF created with pdfFactory Pro trial version www.pdffactory.comEng.

& Tech. Journal, Vol.28, No.15, 2010 Experimental Study Of The Effect Of Addition Of Calming Section On The Heat Transfer Process In A Uniformly Heated Inclined Tube
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& Tech. Journal, Vol.28, No.15, 2010 Experimental Study Of The Effect Of Addition Of Calming Section On The Heat Transfer Process In A Uniformly Heated Inclined Tube
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] Hussein A. Mohammed and Yasin K. Salman, "
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