Forced Convection Heat Transfer Concepts

Newton's law of cooling expresses the rate of convection heat transfer as $Q_{conv} = hA(T_s - T_A)$, indicating it is proportional to the temperature difference.

The typical range of convection heat transfer coefficients for forced convection of gases is between 25 and 250 W/m² °C.

It is difficult due to boundary layer separation and time-dependent flow instabilities that occur, making analytical calculations complex.

<p>The heat transfer rate for gases is primarily influenced by the Reynolds number since the Prandtl number remains practically constant under many conditions.</p> Signup and view all the answers

<p>Thermal boundary layer development is significant as it affects the heat transfer characteristics in configurations like cylinders and tube bundles.</p> Signup and view all the answers

<p>The experimental setup consists of a Perspex working section where air flows past cylindrical elements, simulating cross flow conditions.</p> Signup and view all the answers

<p>Correlations of experimental data are used to relate the Nusselt number to Reynolds number and Prandtl number due to the difficulty in making direct calculations.</p> Signup and view all the answers

<p>The rod used in the setup is made of pure copper, approximately 10 cm in length, attached to extension rods of a fabric-based plastic compound.</p> Signup and view all the answers

<p>Natural convection occurs due to buoyancy forces from temperature differences, while forced convection involves external means, such as fans or pumps, to circulate the fluid.</p> Signup and view all the answers

<p>As flow velocity increases, the heat transfer coefficient also increases due to enhanced fluid motion and reduced thermal resistance.</p> Signup and view all the answers

<p>The Nusselt number (Nu) represents heat transfer, and the Reynolds number (Re) characterizes fluid flow.</p> Signup and view all the answers

<p>Conduction transfers energy from the solid surface to the adjacent fluid, while convection carries that energy away through fluid motion.</p> Signup and view all the answers

<p>When the temperature difference is minimal, heat transfer is primarily by conduction rather than convection.</p> Signup and view all the answers

<p>Comparing these relationships allows for validation of experimental methods and can help improve understanding of heat transfer phenomena.</p> Signup and view all the answers

<p>Bulk fluid motion enhances heat transfer by removing heated fluid near the surface and replacing it with cooler fluid.</p> Signup and view all the answers

<p>A tube bundle consists of multiple tubes arranged to enhance fluid flow around them, and measuring heat transfer in these configurations is significant for improving efficiency in heat exchangers.</p> Signup and view all the answers

<p>The static pressure drop across the tube banks is about four times the velocity head.</p> Signup and view all the answers

<p>The inclined manometer has a range of approximately 75 mm water gauge.</p> Signup and view all the answers

<p>The thermocouples are made from copper and constantan.</p> Signup and view all the answers

<p>The specific heat of the copper element is 380 J/kg.K.</p> Signup and view all the answers

<p>The diameter of the elements is 12.5 mm.</p> Signup and view all the answers

<p>The temperature characteristic is approximately linear, where 1 °C corresponds to 0.041 mV.</p> Signup and view all the answers

<p>The Nusselt number is given by the formula $Nu = \frac{hd}{k}$.</p> Signup and view all the answers

<p>The longitudinal pitch of the elements is 18.75 mm.</p> Signup and view all the answers

<p>The manometer measures the pressure drop between the atmosphere and the upstream static pressure to calculate velocity head.</p> Signup and view all the answers

<p>Standardizing ensures accurate temperature readings of the heated element during the experiment.</p> Signup and view all the answers

<p>The air inlet temperature and total head tube reading need to be recorded.</p> Signup and view all the answers

<p>Reinstalling the heated element allows for continuous measurement of cooling curves at varying temperatures.</p> Signup and view all the answers

<p>The stopwatch is used to time how long it takes for the galvanometer needle to pass through the zero position.</p> Signup and view all the answers

<p>It provides insight into how airflow affects the cooling rate of the heated element.</p> Signup and view all the answers

<p>The pressure drop can be correlated to upstream velocity head, providing a more accurate measure of flow velocity.</p> Signup and view all the answers

<p>Measuring velocity distribution helps analyze flow characteristics and assesses the performance of the working section.</p> Signup and view all the answers

<p>The effective length correction of 8.4 mm accounts for the additional surface area contributed by the plastic extensions, ensuring accurate heat transfer calculations.</p> Signup and view all the answers

<p>The heat transfer coefficient (h) is estimated by plotting the logarithmic relationship of temperature drop against time and calculating the slope of the resulting straight line.</p> Signup and view all the answers

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

<p>The fall in temperature (dT) is proportional to the heat transfer rate (q) and is given by the equation -q.dt = m.c.dT, indicating energy transfer across mass and specific heat.</p> Signup and view all the answers

<p>A higher heat transfer coefficient (h) results in a faster rate of temperature change, meaning the element cools more quickly.</p> Signup and view all the answers

<p>Integrating equation (3) helps derive a relationship that can be analyzed to understand temperature decay over time and estimate the heat transfer coefficient.</p> Signup and view all the answers

<p>Log_e is used in the original equations for accurate relationship modeling, while log_10 is applied on semi-log paper, which affects the interpretation of the plotted data.</p> Signup and view all the answers

<p>Negligible temperature gradients imply that the temperature measured at the center accurately reflects the effective surface temperature, simplifying the analysis.</p> Signup and view all the answers

<p>The heat transfer coefficient is related by the expression $h = -2.3026\frac{m.c}{A_1}M$.</p> Signup and view all the answers

<p>The dynamic pressure (ΔP) represents the pressure difference related to the velocity of the air, given by $ΔP = \frac{ρV_1^2}{2}$.</p> Signup and view all the answers

<p>The relationship is $ρV_1^2 = 98.1 H$, where ΔP is measured in terms of head difference.</p> Signup and view all the answers

<p>When all tubes are present, the effective velocity is given by $V = 2V_1$.</p> Signup and view all the answers

<p>The Prandtl number is significant as it remains constant for gases under a wide range of conditions, affecting heat transfer rates.</p> Signup and view all the answers

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

<p>The formula is $V_1 = 237.3\sqrt{\frac{H.T_A}{p_A}}$.</p> Signup and view all the answers

<p>The minimum flow area is (9/10) of the full working section area, leading to the expression $V = \frac{10}{9}V_1$.</p> Signup and view all the answers