Forced Convection Heat Transfer Quiz
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Questions and Answers

What does Newton's law of cooling express about convection heat transfer?

  • It is only applicable to gases.
  • It is independent of temperature difference.
  • It increases with the surface area only.
  • It is proportional to the temperature difference. (correct)

    • Which type of convection typically has the highest convection heat transfer coefficient?

  • Free convection of gases
  • Forced convection of liquids
  • Free convection of liquids
  • Boiling and condensation (correct)

    • What is a key factor in determining the heat transfer characteristics of cross flow configurations?

  • The conductivity of the material
  • Type of fluid in the tubes
  • The size of the tubes used
  • Thermal boundary layer development (correct)

    • Which number relates the Nusselt number to Reynolds number and Prandtl number?

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

    What factor is practically constant for gases under a wide range of conditions?

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

    What is the purpose of the Perspex rods in the experimental setup?

    <p>To simulate a cross flow heat exchanger</p> Signup and view all the answers

    What is a limitation in calculating average heat transfer coefficients in cross flow?

    <p>3-D and time dependent flow instabilities</p> Signup and view all the answers

    What is the preferred method to indicate air velocity past the tube bank?

    <p>Observing pressure drop</p> Signup and view all the answers

    What is the maximum pressure difference and velocity head achievable in the apparatus?

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

    Which of the following best describes the convection heat transfer coefficient for forced convection of gases?

    <p>25-250 W/m² °C</p> Signup and view all the answers

    What is the temperature characteristic of the thermocouples within the specified range?

    <p>Approximately linear</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

    Which dimension corresponds to the diameter of the tube elements?

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

    What is the formula for calculating the Reynolds number?

    <p>$Re = \frac{ρVd}{μ}$</p> Signup and view all the answers

    What type of manometer can be used for greater precision in measuring pressure drop?

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

    Which dimension represents the surface area of the element?

    <p>$A = \frac{d^2}{4} \times \pi$</p> Signup and view all the answers

    What is the maximum temperature that the copper element reaches when heated?

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

    Which instrument measures the temperature difference within the copper element?

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

    What purpose does the honeycomb flow straightener serve in the apparatus?

    <p>To prevent swirl from the fan</p> Signup and view all the answers

    How is the velocity through the apparatus regulated?

    <p>By using a manual throttle valve</p> Signup and view all the answers

    What feature allows for the exploration of the flow pattern upstream of the tube bank?

    <p>Total head tube</p> Signup and view all the answers

    What is the purpose of the static tapings associated with the apparatus?

    <p>To record the velocity head</p> Signup and view all the answers

    What initial measurement is indicated at the inlet of the air stream?

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

    What is a function of the centrifugal fan in the apparatus?

    <p>To regulate air velocity through the working section</p> Signup and view all the answers

    What happens to the Nusselt number as the Reynolds number increases in forced convection scenarios?

    <p>It increases, indicating improved heat transfer.</p> Signup and view all the answers

    Which measurement is required for calculating the velocity head upstream of the element?

    <p>Upstream water height in cm H<sub>2</sub>O</p> Signup and view all the answers

    When the radiator in a car shows a dangerous temperature after stopping, what could be a potential explanation?

    <p>Insufficient airflow over the radiator during idling.</p> Signup and view all the answers

    In the context of heat transfer, what does the relationship TA refer to?

    <p>Ambient temperature of the surrounding environment.</p> Signup and view all the answers

    What is one way to characterize forced convection in a system?

    <p>Movement of the fluid is created by external means like fans or pumps.</p> Signup and view all the answers

    What assumption is made about the temperature gradients within the cylindrical copper element?

    <p>They are negligible, allowing accurate temperature measurement.</p> Signup and view all the answers

    What correction is applied to the true length of the copper element?

    <p>8.4 mm</p> Signup and view all the answers

    Which variable in the equation for heat transfer rate ($q=hA(T-T_A)$) represents the temperature difference?

    <p>$(T-T_A)$</p> Signup and view all the answers

    How is the heat transfer coefficient ($h$) ultimately determined?

    <p>By measuring the slope of a plot of log data against time.</p> Signup and view all the answers

    Which equation indicates 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

    What is plotted on a semi-log paper to estimate the heat transfer coefficient?

    <p>$ rac{T-T_A}{T_o-T_A}$ vs $t$</p> Signup and view all the answers

    When integrating the temperature change equations, what form does the equation take?

    <p>$log_e rac{T-T_A}{T_o-T_A} = - rac{hA_1t}{m.c}$</p> Signup and view all the answers

    In the context of heat transfer, what does the variable $(T_o)$ represent?

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

    How is the heat transfer coefficient (h) related to the slope (M) of the line on semi-log paper?

    <p>$h = -2.3026\frac{m.c}{A_1}M$</p> Signup and view all the answers

    What is the equation for dynamic pressure measured by a Pitot tube?

    <p>$\Delta P = \frac{\rho V_1^2}{2}$</p> Signup and view all the answers

    What is the value of the gas constant R for the density equation?

    <p>287 J/kg.K</p> Signup and view all the answers

    According to the equations, what is the effective velocity (V) based on the condition of studying a single element in isolation?

    <p>$V = \frac{10}{9}V_1$</p> Signup and view all the answers

    Under what condition is the Prandtl (Pr) number considered constant?

    <p>For gases under a wide range of conditions</p> Signup and view all the answers

    What does the relationship $\frac{hd}{k} = f( \frac{pVd}{\mu},\frac{\mu C_p}{k})$ signify in dimensional analysis?

    <p>A variable heat transfer coefficient based on velocity</p> Signup and view all the answers

    What does equation $Nu = f(Re.Pr)$ suggest about the heat transfer?

    <p>It is influenced by both Reynolds and Prandtl numbers</p> Signup and view all the answers

    What is the corresponding dynamic pressure formula in relation to head difference (ΔH) measured by a manometer?

    <p>ΔP = ρgΔH</p> Signup and view all the answers

    Study Notes

    Forced Convection Heat Transfer

    • 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 in dimensionless form (Nusselt number vs. Reynolds number) and compare to existing correlations.
      • Repeat measurements, correlation, and comparison for a tube bundle.

    Background

    • Convection: Energy transfer between a solid surface and a moving fluid (liquid or gas). This involves combined conduction and fluid motion.
      • Faster fluid motion leads to greater heat transfer by convection.
      • In the absence of bulk fluid movement, heat transfer is purely by conduction.
      • Fluid motion enhances heat transfer with a solid surface but also makes determining rates more complex.
    • Forced Convection: Fluid movement is forced by external means (e.g., fan, pump, wind).
    • Natural (Free) Convection: Fluid movement is caused by buoyancy forces due to density differences from temperature variations within fluid.

    Experimental Setup

    • Apparatus: Perspex working section with air flow by a centrifugal fan.
      • Perspex rods can be inserted to simulate cross-flow heat exchangers.
      • A copper element is used for heat transfer studies within working section.
    • Instrumentation:
      • Thermocouples: Measures the element and air inlet temperatures.
      • Manometer/Micro-manometer: Measures pressure difference & velocity head.
      • Potentiometer: Records temperature difference for calculations.

    Theoretical Background

    • Newton's Law of Cooling: The rate of convection heat transfer is proportional to the temperature difference.
    • Heat Transfer Coefficient: Rate of transmission of heat from the element to air (h).
    • Equation for Heat Transfer Coefficient: An equation relating temperature change over time to heat transfer coefficient (using log).
    • Calculating Velocity: The velocity of air is measured using a pitot tube, dynamic pressure (AP), and a manometer
    • Minimum Flow Area: Critical flow area consideration when calculating velocities through a tube bank.

    Experimental Procedure

    • Detailed steps for setting up the apparatus and conducting the convection experiments.
      • Setup procedures include manometer connections, element placement, and flow rate control.
      • Specific procedures for recording measurements, plotting cooling curves and using heat transfer coefficient calculations.

    Results and Analysis

    • Data: Collected data on temperature, time, flow rate...
    • Calculations: Calculate heat transfer coefficients using the obtained data.
    • Comparison: Comparison of calculated heat transfer coefficients with existing correlations / established theoretical models.

    Additional Information

    • (Specimen data, tables)

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    Description

    Test your understanding of forced convection heat transfer, its differences from natural convection, and how to measure the heat transfer coefficient in tube systems. This quiz will also cover the presentation of experimental measurements and comparisons with existing correlations.

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