Lab. Experiment: Heat Transfer for Fluid Flow in a Pipe
The aim of the experiment was to look into heat transportation between the interior surface of a pipe maintained at unvarying temperature and a fluid fluxing along the pipe. Consequences were obtained for all the four openings. Air flow rates were measured for the different diameter openings and the force per unit area of air flow was measured. From, the consequences obtained we were able to demo the relation between Stanton and Reynolds figure and compare it with the Dittus Boelter correlativity
Heat transportation in industrial heat money changers normally occurs between the surface of the pipe and a fluid fluxing inside it. It is hence of import to be able to cipher the heat-transfer coefficients for flow inside pipes so that the entire heat transportation rate and size of the equipment can be calculated. [ 1 ] In the experiment, an probe on the heat transportation to air fluxing through a pipe with a changeless wall temperature was carried out.
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Background and Theory
Convection heat transportation between the interior surface of a pipe and a fluid is relative to the speed of the fluid. The higher the speed, the larger the flow rate and the higher the heat transportation rate ( Cengel, 2003 ) .
As the fluid enters the pipe with unvarying speed, the fluid adjacent to the wall is brought to rest. The boundary bed additions in thickness until it fills the whole pipe and from this point the speed profile along the canal remains unchanged. If the turbulency in the entrance fluid is high, the boundary bed will go disruptive. Forced convection in the pipe increases the heat transportation rate. [ 3 ]
The heat transportation coefficient in the entryway part of the pipe will be given to be higher than the remainder of the canal. When a fluid with unvarying speed enters the pipe a speed boundary bed, ( besides a thermic boundary bed if heated ) starts developing along the surface of the pipe. The heat transportation through a pipe is dependent on the thickness of the pipe and isolation beds it contains.
The flow beyond the entrance part of the pipe is termed “fully developed flow” . The passage from laminar to turbulent flow is likely to happen in the entryway part.
For a fluid, such as air fluxing through a pipe, the heat transportation coefficient and therefore the Nusselt figure depend chiefly on the Reynolds figure. For Re=2000, flow is said to be laminal, heat transportation is by conductivity merely, and heat-transfer coefficients are comparatively low. For Re ~ 10 000, the flow is disruptive and the commixture of hot and cold Eddy fluids leads to higher heat-transfer coefficients than in the laminal part.
( 1 ) Heat transportation coefficient is expressed in footings of the dimensionless Nusselt figure, Nu as:
( 2 ) Dimensional analysis indicates the being of the undermentioned relationship for forced convection heat transportation for flow in pipes.
Dittus Boelter correlativity is applicable when forced convection is the lone manner of heat transportation.
Dittus Boelter correlativity:
( 3 ) Apparatus
* Numbers indicate thermocouples
The setup shown in Fig.2 consists of a steel pipe with a merriment to pull air through it. The pipe is fitted with five electric warmers, maintained at a changeless wall temperature over a subdivision of 1.5 m long. Thermocouples are used to mensurate the wall temperatures at 10 points along the het subdivision and the recess and mercantile establishment temperatures of the air. The insulated commixture chamber insures that the mensural mercantile establishment temperature is near to the true “bulk” average temperature. The pipe is divided into five subdivisions along its het length, so that the fluctuation in heat-transfer coefficient along the pipe can be investigated. A venturi metre, with a H2O manometer, is used to mensurate the mass flow rate of the air.
· Atmospheric force per unit area and the ambient temperature were recorded.
· Manometer reading was recorded when connected between the recess and pharynx of the Venturi metre and when connected between venturi recess and ambiance.
· Temperatures were recorded for each of the five warmers in the pipe.
· The power dissipated was recorded from the picker switch, in each of the five warmers as a per centum of their sum rated power of 1200 W.
· From the 2nd picker switch, the recess and mercantile establishment air temperatures were obtained.
· The opening on the fan issue was removed, after a sufficient clip ( ~20 proceedingss ) , when all the wall temperatures and heater powers became steady, to let a 2nd set of observations to obtained.
· In entire, consequences for four flow rates were obtained ( one with to the full unfastened opening, the other three with different opening home bases ) .
Atmospheric force per unit area = 772.50 mmHg = 102991.5 Pa
Ambient temperature = 21 oC = 294.15 K
Wall temperature ( Heaters ) : H1=80, H2=81, H3=80, H4=80, H5=80 ( changeless )
Inlet Temp. = 19 oC
Outlet Temp. = 38 oC
Table 1: Experimental Consequences
Venturi recess to throat/ mmH2O
recess to atm. /mmH2O
Venturi recess to throat/Pa
recess to atm. /Pa
Fully unfastened ( 62 )
H1=26 H2=13 H3=15 H4=16 H5=16
H1=22 H2=12 H3=15 H4=15 H5=14
H2=11 H3=13 H4=13 H5=13
Sample Calculations and Consequences
Case: Fully unfastened opening – 62 millimeter
Air mass flow rate
Heater powers / W
Log-mean temperature differences
Surface heat-transfer coefficients
The aim of the experiment was to look into heat transportation between the interior surface of a pipe maintained at unvarying temperature and a fluid fluxing along the pipe.
Reynolds figure, Re was calculated for all the four openings, and found that the scope of values were between ~20 000 – 40 000, guaranting turbulent flow and cogency of the Dittus Boelter correlativity, since forced convection was present in the pipe. The heat transportation coefficient and the Nusselt Number depend chiefly on the Reynolds figure. Prandtl Number, Pr is merely dependent on the fluid and the fluid province, unlike Reynolds figure where it is subjected to length graduated table variable. The Pr for the air flow in the three instances was changeless and little in value, leting the heat to spread rapidly. Variation in Pr would take to different viscousnesss in the warmers and hence affect the result of the consequences.
Fluid flow can be divided into three parts. Below Re=2000, the flow is laminal, heat transportation is by conductivity merely and heat transportation coefficients are comparatively low. For Re~10 000, the flow is disruptive and commixture of hot and cold fluid by eddy gesture leads to higher heat transportation coefficients than in the laminal part. Between the laminar and disruptive parts is a passage part where a little sum of commixture occurs. Entrance effects are non accounted for the Dittus Boelter correlativity as it is merely applicable for forced convections, i.e. disruptive flow.
The chief mistakes carried out in the experiment were due to human mistakes and setup used. Parallax mistakes were one of the chief factors in the experiment, where taking readings from the manometer contributed significantly to experimental mistakes.
The truth of the Venturi metre, barometer, manometer and thermocouples besides accounted for the possible mistakes in the experiment. Mistakes resulted in less accurate readings in ciphering the heat-transfer rates, heat transportation coefficient, mass flow rate, air denseness etc. This is because there would hold been mistakes in the ambient temperature, wall temperature, atmospheric temperature, warmer powers ( % ) etc. Having more warmers along a greater part in the pipe would give better fluctuations in the heat transportation coefficient.
Due to clip restraints the experiment was merely carried out one time, mistakes could hold been minimised by reiterating the experiment 5 – 10 times. The result of the experiment turned out to be reasonably accurate, although if mistakes were taken into history when ciphering the consequences, the result would hold been better.
The result of the experiment turned out moderately good. Consequences were obtained for all the four openings. Air flow rates were measured for the different diameter openings and the force per unit area of air flow was measured. From, the consequences obtained we were able to demo the relation between Stanton and Reynolds figure and compare it with the Dittus Boelter correlativity, and found out that the values followed the correlativity.