Forced Convection Heat Transfer Quiz
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Questions and Answers

What is the formula to calculate the velocity head, upstream the element, H1?

  • H<sub>1</sub> = m * g
  • H<sub>1</sub> = p<sub>A</sub>/2
  • H<sub>1</sub> = 237.3 * T<sub>A</sub>/p<sub>A</sub> (correct)
  • H<sub>1</sub> = 874.988 * (m.M/A<sub>1</sub>)

    • What unit is used for the Nusselt number as referenced in the lab?

  • K
  • Dimensionless (correct)
  • m/s
  • W/m<sup>2</sup>.K

    • What is the mass flow rate representation in the context provided?

  • ρV<sub>1</sub> (correct)
  • p<sub>A</sub> * V<sub>1</sub>
  • m<sub>1</sub>
  • M<sub>1</sub>

    • Which factor primarily affects the heat transfer coefficient calculated in the lab?

    <p>The nature of the fluid flow</p> Signup and view all the answers

    Why does the temperature gauge in a car show dangerous levels when stopped at a traffic light?

    <p>There is reduced airflow over the radiator</p> Signup and view all the answers

    What is the approximate relationship between the static pressure drop and the velocity head across the four tube banks?

    <p>It is about four times the velocity head.</p> Signup and view all the answers

    Which type of thermocouples is used in the described apparatus?

    <p>Copper and constantan</p> Signup and view all the answers

    What is the maximum pressure difference and velocity head that the apparatus can measure?

    <p>75 mm water gauge</p> Signup and view all the answers

    What is the specific heat of the copper element used in the apparatus?

    <p>380 J/kg.K</p> Signup and view all the answers

    What is the diameter of the elements used in the setup?

    <p>12.5 mm</p> Signup and view all the answers

    What does the Nusselt number (Nu) represent in the context of this apparatus?

    <p>The ratio of convection to conduction heat transfer</p> Signup and view all the answers

    Which device can provide greater precision in measuring low pressure differences in the apparatus?

    <p>Electronic micro manometer</p> Signup and view all the answers

    At what temperature range does the temperature characteristic of the thermocouple behave approximately linearly?

    <p>0-50°C</p> Signup and view all the answers

    What is used to record the temperature of the copper element during the heating process?

    <p>Thermocouple potentiometer</p> Signup and view all the answers

    What maximum temperature can the copper element reach when heated?

    <p>80 °C</p> Signup and view all the answers

    What component is responsible for preventing swirl from the fan into the working section?

    <p>Honeycomb flow straightener</p> Signup and view all the answers

    How is the air velocity through the apparatus regulated?

    <p>Using a graduated throttle valve</p> Signup and view all the answers

    What is the function of the total head tube in the apparatus?

    <p>To survey flow pattern upstream of the tube bank</p> Signup and view all the answers

    What is indicated by the mercury-in-glass thermometer in the apparatus?

    <p>Initial temperature of the air</p> Signup and view all the answers

    What is the main purpose of the centrifugal fan in the apparatus?

    <p>To control air velocity and flow</p> Signup and view all the answers

    How is the heat transfer coefficient between the copper element and the air calculated?

    <p>With known values of thermal capacity and surface area</p> Signup and view all the answers

    What is the purpose of connecting the manometer to the total head tube?

    <p>To determine the velocity head upstream of the working section</p> Signup and view all the answers

    What should be done after reaching the temperature of about 70°C in the element?

    <p>Replace the element in the working section</p> Signup and view all the answers

    What is indicated by the depression at the static tapping?

    <p>Value of H<sub>1</sub></p> Signup and view all the answers

    Why is it important to observe the galvanometer needle passing through the zero position?

    <p>To time the decrease in thermocouple voltage accurately</p> Signup and view all the answers

    What is the significance of plotting cooling curves for different air velocities?

    <p>To analyze the performance of the heating element under varying conditions</p> Signup and view all the answers

    What is the role of the throttle valve in this setup?

    <p>To adjust the flow rate of air through the system</p> Signup and view all the answers

    What does the relationship between upstream velocity head and pressure drop across the tube bank help determine?

    <p>More accurate measurements of flow velocity</p> Signup and view all the answers

    What should be done with the potentiometer before observing the galvanometer needle?

    <p>Adjust it to a reading lower than that of the element temperature</p> Signup and view all the answers

    What is the main assumption about the heat lost from the cylindrical copper element?

    <p>Heat lost is entirely transferred to the air flowing past it.</p> Signup and view all the answers

    What is the relationship between the heat transfer coefficient, h, and the slope, M, of the line in a semi-log plot?

    <p>h = -2.3026 m.c/A_1 * M</p> Signup and view all the answers

    How can the velocity of the air, V_1, be calculated according to the content provided?

    <p>V_1 = 237.3 sqrt(H * T_A / p_A)</p> Signup and view all the answers

    How is the effective length of the copper element calculated?

    <p>By making a correction of 8.4 mm to the true length of the element.</p> Signup and view all the answers

    What is the effective area when all tubes are present in a transverse plan including a row of 5 tubes?

    <p>Effective area is one half that of the working section</p> Signup and view all the answers

    Which equation represents the rate of heat transfer from the element to the air?

    <p>$q = h<em>A</em>(T-T_A)$</p> Signup and view all the answers

    What does the variable $(T_o)$ represent in the equations?

    <p>The initial temperature of the element at time t=0.</p> Signup and view all the answers

    In the context of convective heat transfer, which variable primarily affects the rate of heat transfer for gases?

    <p>Reynolds number</p> Signup and view all the answers

    What does the slope of the line in the integrated equation indicate?

    <p>The heat transfer coefficient (h).</p> Signup and view all the answers

    What does the equation $ΔP = ρV_1^2 / 2$ represent in the context provided?

    <p>The dynamic pressure measured with a manometer</p> Signup and view all the answers

    In which scenario is the minimum flow area expressed as (9/10) of the full working section area?

    <p>When isolated elements are studied</p> Signup and view all the answers

    What should be plotted on semi-log paper to observe the cooling behavior?

    <p>[(T-T_A)/(T_o-T_A)]</p> Signup and view all the answers

    Which formula shows the dimensionless number relationship for convective heat transfer?

    <p>$Nu = f(Re)$</p> Signup and view all the answers

    Which of the following shows the relationship between temperature change and time?

    <p>$ rac{dT}{dt} = - rac{hA_1}{m.c}(T-T_A)$</p> Signup and view all the answers

    In calculating heat transfer, what is assumed about temperature gradients within the copper element?

    <p>They are negligible.</p> Signup and view all the answers

    What does $p_A = R.T_A/P$ describe?

    <p>The density of the gas under specific conditions</p> Signup and view all the answers

    Study Notes

    Forced Convection Heat Transfer to a Tube in Cross Flow

    • Objectives:
      • Understand the difference between natural and forced convection.
      • Measure heat transfer coefficient for a tube in cross flow as a function of flow velocity.
      • Present experimental measurements using dimensionless form (Nusselt number (Nu) versus Reynolds number (Re)).
      • Compare measured Nu-Re relation with existing correlations.
      • Repeat measurements and comparison for a tube bundle.

    Background

    • Convection: Mode of energy transfer involving conduction and fluid motion.
      • Faster fluid motion = greater heat transfer by convection.
      • In absence of fluid motion, heat transfer is purely conductive.
      • Fluid motion enhances heat transfer between solid surface and fluid, but also makes determining rates more complex.
    • Forced Convection: Fluid motion caused by external means (fan, pump, wind).
    • Natural (Free) Convection: Fluid motion caused by buoyancy forces due to density differences (temperature variations).
    • Energy Transfer:
      • Energy first transferred to air layer adjacent to the block by conduction.
      • Convection (conductive motion in the air and macroscopic motion removing warm air and replacing with cool air) carries heat away from the surface.

    Theory

    • Newton's Law of Cooling: Rate of Convection heat transfer is proportional to the temperature difference.
      • Q = h * A * (Ts - Tf)
        • where: h = convection heat transfer coefficient A = surface area Ts = surface temperature Tf = fluid temperature.
    • Typical Convection Heat Transfer Coefficient Values:
      • Free convection of gases: 2-25 W/m² °C
      • Free convection of liquids: 10-1000 W/m² °C
      • Forced convection of gases: 25-250 W/m² °C
      • Forced convection of liquids: 50-20000 W/m² °C
      • Boiling and condensation: 2500-100000 W/m² °C

    Experimental Setup

    • Apparatus: Perspex working section, centrifugal fan, total head tube, manometer, thermocouples, copper element, plastic extensions.
    • Temperature Measurement: Thermocouple, potentiometer, mercury-in-glass thermometers.
    • Air Velocity Measurement: Pitot tube.
    • Pressure Difference: Inclined manometer or electronic micro manometer.

    Experimental Procedure

    • Detailed procedure for setting up, running, and recording data in order to obtain cooling curves for the element under various flow conditions, to determine heat transfer coefficient values.

    Test Sheet & Calculations

    • Examples of data tables (including temperature, time, and velocity).
    • Relevant calculations and formulas required for determining heat transfer values.
    • Important Note: Detailed calculations, relationships, and data analysis, are not available in this summarization. These steps should occur after data collection.

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    Description

    Test your understanding of forced convection heat transfer to a tube in cross flow. This quiz covers the measurements of heat transfer coefficients, the significance of flow velocity, and the Nusselt number versus Reynolds number relations. Prepare to compare your results with existing correlations and understand the underlying concepts of convection.

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