Forced Convection Heat Transfer Quiz
45 Questions
4 Views

Choose a study mode

Play Quiz
Study Flashcards
Spaced Repetition
Chat to Lesson

Podcast

Play an AI-generated podcast conversation about this lesson

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 (A)</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 (B)</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. (D)</p> Signup and view all the answers

Which type of thermocouples is used in the described apparatus?

<p>Copper and constantan (D)</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 (C)</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 (D)</p> Signup and view all the answers

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

<p>12.5 mm (D)</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 (B)</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 (D)</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 (D)</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 (B)</p> Signup and view all the answers

What maximum temperature can the copper element reach when heated?

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

How is the air velocity through the apparatus regulated?

<p>Using a graduated throttle valve (B)</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 (A)</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 (D)</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 (C)</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 (C)</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 (B)</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 (A)</p> Signup and view all the answers

What is indicated by the depression at the static tapping?

<p>Value of H<sub>1</sub> (B)</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 (A)</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 (B)</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 (D)</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 (A)</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 (B)</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. (D)</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 (C)</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) (D)</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. (C)</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 (D)</p> Signup and view all the answers

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

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

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

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

Flashcards

Heating copper element

Copper element is heated separately and then reintroduced to the working section for cooling rate measurement.

Cooling rate measurement

The rate at which the copper element cools is measured by an embedded thermocouple.

Heat transfer coefficient

This coefficient is calculated from the cooling rate data and the element's thermal capacity and surface area.

Electric heater

The copper element is heated using an electric heater that raises the element's temperature up to 80°C.

Signup and view all the flashcards

Thermocouple potentiometer

A device to measure the temperature difference between a hot junction within the element and a cold junction in the air stream.

Signup and view all the flashcards

Air Velocity Regulation

Air velocity through the apparatus is adjusted by a graduated throttle valve.

Signup and view all the flashcards

Flow Pattern Upstream

Measuring flow upstream of a tube bank using a total head tube at multiple points to assess flow uniformity.

Signup and view all the flashcards

Velocity Distribution

The velocity distribution upstream of the tube bank is fairly consistent and is measured using static wall pressure in the downstream area.

Signup and view all the flashcards

Static pressure drop

The pressure difference across a tube bank when air is flowing, and is approximately four times the velocity head.

Signup and view all the flashcards

Velocity head

A measure of the kinetic energy of a fluid, proportional to the square of the fluid's velocity.

Signup and view all the flashcards

Pressure difference

The difference in pressure between two points in a fluid, often used to measure air velocity.

Signup and view all the flashcards

Thermocouple

A device used to measure temperature difference, using copper and constantan elements.

Signup and view all the flashcards

Copper element

A copper component used in the experiment to study heat transfer.

Signup and view all the flashcards

Nusselt number (Nu)

A dimensionless number that represents the ratio of convective to conductive heat transfer.

Signup and view all the flashcards

Reynolds number (Re)

A dimensionless number that represents the ratio of inertial forces to viscous forces.

Signup and view all the flashcards

Prandtl number (Pr)

A dimensionless number that describes the ratio of momentum diffusivity to thermal diffusivity.

Signup and view all the flashcards

What is a total head tube?

A total head tube is a device that measures the total pressure in a fluid flow.

Signup and view all the flashcards

What's the function of the static tapping?

A static tapping is a small hole in the wall of a flow channel, used to measure the static pressure.

Signup and view all the flashcards

What is the velocity head?

The velocity head is the pressure difference between the total head and the static pressure, representing the kinetic energy of the flow.

Signup and view all the flashcards

How is the velocity head measured?

The velocity head is measured by the difference between the total head reading and the static tapping reading.

Signup and view all the flashcards

How is air velocity determined in the experiment?

Air velocity is determined by measuring the pressure drop across the tube bank, which is directly proportional to the square of the air velocity.

Signup and view all the flashcards

Why is the copper element heated?

The copper element is heated to a known temperature and then placed in the airstream. As it cools, the heat transfer rate is measured.

Signup and view all the flashcards

What purpose does the thermocouple serve?

The thermocouple is used to measure the temperature of the copper element as it cools down.

Signup and view all the flashcards

How are cooling curves plotted?

Cooling curves are plotted by observing the temperature of the copper element over time as it cools in the airstream.

Signup and view all the flashcards

Heat transfer coefficient (h)

A proportionality constant representing the rate of heat transfer from a surface to a fluid.

Signup and view all the flashcards

Cooling rate calculation

Method for estimating the heat transfer coefficient by measuring how fast an object cools.

Signup and view all the flashcards

Equation (1) - Heat Transmission

q = h * A * (T - T_A). Calculates heat transmission rate.

Signup and view all the flashcards

Equation (2) - Temperature Change

-q.dt = m.c.dT. Describes temperature change during cooling.

Signup and view all the flashcards

Effective Length Correction

Adding 8.4mm to the actual length in the temperature calculation to account for heat conduction to the plastic extension.

Signup and view all the flashcards

Temperature Gradient Assumption

The assumption that temperature differences within the copper element are negligible; the thermocouple gives an accurate surface temperature reading.

Signup and view all the flashcards

Logarithmic Plot

A plot of logₑ [(T-Tₐ)/(T₀-Tₐ)] against time (t) that results in a straight line whose slope is related to (h).

Signup and view all the flashcards

Assumptions in the Study

All heat loss is to the passing air, and temperature differences inside the component are minimal

Signup and view all the flashcards

Semi-log paper

Graph paper where one axis uses a logarithmic scale and the other uses a linear scale. This allows for visual representation of exponential relationships.

Signup and view all the flashcards

Effective velocity (V)

The average velocity of the fluid passing through a specific area, often calculated based on the minimum flow area.

Signup and view all the flashcards

Pitot tube

A device used to measure fluid velocity by converting kinetic energy into pressure.

Signup and view all the flashcards

Velocity head (ΔH)

The pressure difference caused by the kinetic energy of a moving fluid, often measured with a manometer.

Signup and view all the flashcards

Minimum flow area

The smallest cross-sectional area through which the fluid must pass, often used to calculate the effective velocity.

Signup and view all the flashcards

Forced Convection

Heat transfer mechanism where fluid motion is induced by external means, like a pump or fan, rather than by natural buoyancy forces. This results in enhanced heat transfer.

Signup and view all the flashcards

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.

Studying That Suits You

Use AI to generate personalized quizzes and flashcards to suit your learning preferences.

Quiz Team

Related Documents

Report Forced Convection Heat Transfer PDF

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.

More Like This

Use Quizgecko on...
Browser
Open
Browser
Browser