Heat Transfer and Convection Principles

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Questions and Answers

What is the likely reason for the increased temperature reading in a car radiator when stopped?

The coolant does not circulate properly while stopped.

The airflow over the radiator decreases, reducing heat dissipation. (correct)

The engine temperature is the same regardless of speed.

The engine produces less heat at low speeds.

Which dimensionless number is crucial for determining the heat transfer coefficient in fluid flow?

Grashof number

Mach number

Prandtl number

Nusselt number (correct)

What is the primary principle behind a thermocouple?

It measures pressure by converting thermal energy into mechanical energy.

It uses a single metal wire to measure changes in resistance with temperature.

It calculates temperature based on fluid flow rates.

It generates a voltage based on the temperature differential between two different metal conductors. (correct)

What is the significance of the Reynolds number in fluid flow analysis?

It determines whether the flow is laminar or turbulent. Signup and view all the answers

In designing an experimental setup to measure pressure drop, what is a critical consideration?

Fluid type and flow rate must be controlled. Signup and view all the answers

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

The static pressure drop is four times the velocity head. Signup and view all the answers

Which of the following statements about thermocouples in the apparatus is true?

They provide a voltage of 0.041 mV for every 1°C change within the range 0-50°C. Signup and view all the answers

What is the role of the integral cabinet in the experimental setup?

It carries an isolator, starter, and control switch for the heater. Signup and view all the answers

Which dimensionless number is used to characterize the ratio of momentum diffusivity to thermal diffusivity?

Prandtl number, Pr Signup and view all the answers

In the experimental setup, which is indicated as the maximum pressure difference possible?

75 mm water gauge Signup and view all the answers

Which of the following best explains the function of the Nusselt number?

It quantifies the heat transfer coefficient. Signup and view all the answers

What material are the tube elements made of as per the specifications?

Copper Signup and view all the answers

Which characteristic of the thermocouple's temperature measurement is emphasized in the context of this setup?

It displays a linear characteristic within a specific temperature range. Signup and view all the answers

What is measured using the depression at the static tapping during the experiment?

The velocity head upstream of the working section Signup and view all the answers

Which of the following is the first step in setting up the apparatus?

Setting the heated element in the desired position Signup and view all the answers

What is the primary purpose of standardizing the thermocouple potentiometer?

To correlate temperature readings with voltage output Signup and view all the answers

How is the cooling curve obtained during the experiment?

By recording stopwatch readings during potentiometer adjustments Signup and view all the answers

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

To determine the relationship between flow velocity and heat transfer Signup and view all the answers

Which measurement is essential for calculating the flow velocity in the preliminary experiment?

The pressure drop across the tube bank Signup and view all the answers

In the experiment, what happens when the temperature of the element reaches around 70°C?

The element is replaced back into the working section Signup and view all the answers

What should be observed in the galvanometer while setting the potentiometer to lower values?

The needle passes through the zero position 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 data in dimensionless form (Nusselt number (Nu) vs. Reynolds number (Re)).
- Compare measured Nu-Re relation with existing correlations.
- Repeat measurements, correlation, and comparison for a tube bundle.

Background

Convection: Energy transfer between a solid surface and adjacent liquid/gas in motion.
- Involves combined effects of conduction and fluid motion.
- Faster fluid motion = greater heat transfer by convection.
- Without bulk fluid motion, heat transfer is by conduction only.
- Fluid motion enhances heat transfer between solid surface and fluid but also complicates rate determination.
Types of Convection:
- Forced: Fluid flow is forced (e.g., by a fan, pump, or wind).
- Natural (or free): Fluid motion is caused by buoyancy forces due to density differences from temperature variations.

Experimental Setup

Apparatus: Perspex working section with air drawn by a centrifugal fan.
- Rods (copper) inserted to simulate cross-flow heat exchanger.
- Element is heated (copper rod) within working section.
- Thermocouples measure temperatures
- Manometer used to measure pressure difference
Measurement of Velocity:
- Pitot tube used to measure air velocity upstream of the tube bank.
- Dynamic pressure measured (AP) which is related to velocity.

Theoretical Background

Newton's Law of Cooling:
- Convection heat transfer rate is proportional to temperature difference.
- Qconvection = hA(T_surface - T_fluid).
- h is the heat transfer coefficient (W/m² °C).
Heat Transfer Coefficient (h):
- Determined by measuring how quickly the element cools.
- Plot of log [(T-T_a)/(T_o-T_a)] vs. time (t) gives a straight line and allows calculation of h.

Experimental Procedure

Detailed procedure of assembling and operating the apparatus.
- Step-by-step process to measure the process and record the data

Test Sheet/Data Collection

Instructions on how to record ambient temperature, element dimensions, voltage, time, and pressure readings.

Required Work

Data analysis and report writing instruction relating to the analysis of the data collected and comparison with existing data

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Description

This quiz explores the concept of forced convection, focusing on heat transfer characteristics in a tube under cross flow conditions. You'll learn to measure the heat transfer coefficient and present data using dimensionless numbers like Nusselt and Reynolds numbers. Understand the differences between natural and forced convection and the significance of fluid motion in heat transfer.