Question

During a summer day a scooter rider feels more comfortable while on the move than while at rest at a stop light because

• A. More heat is lost by convection while in motion
• B. Air is transparent to radiation, hence it is cooler than the body
• C. The object in motion captures less solar radiation
• D. Air has a low specific heat, hence it is cooler

Hint:

Forced Convection vs. Natural Convection: - Stationary state $\implies$ Natural Convection $\implies$ Low velocity $\implies$ Small $h$ value $\implies$ Poor heat dissipation. - Moving state $\implies$ Forced Convection $\implies$ High relative velocity $\implies$ Large $h$ value $\implies$ High heat dissipation rate.

Answer 1

The Correct Option is A

Concept: The thermal sensation of comfort experienced by a human body in a warm environment depends on the rate of heat dissipation from the skin to the surrounding air. The human body continuously generates metabolic heat, which must be rejected to prevent body temperature from rising. Heat transfer from the skin occurs primarily via three mechanisms: conduction, radiation, and convection. Convective heat transfer (\(q\)) between a surface (the rider's body) and a moving fluid (air) is modeled by Newton's Law of Cooling: \[ q = h \cdot A \cdot (T_{\text{skin}} - T_{\text{ambient}}) \] Where:

• \(h\) = Convective heat transfer coefficient.

• \(A\) = Exposed surface area of the body.

• \(T_{\text{skin}}\) = Surface temperature of the rider's skin.

• \(T_{\text{ambient}}\) = Temperature of the surrounding air on a summer day.
Let us evaluate the two distinct operational states described:

Step 1: Evaluating the rider state at rest at a stop light.
When the scooter is stationary, the air surrounding the rider is nearly still. Heat transfer occurs via natural (free) convection, where fluid motion is driven solely by buoyancy forces arising from temperature-induced density gradients near the skin. The convective heat transfer coefficient for natural convection (\(h_{\text{natural}}\)) is low, limiting heat dissipation and causing the rider to feel warm.

Step 2: Evaluating the rider state while on the move.
When the scooter accelerates into motion, the relative velocity between the rider's body and the air increases substantially. This shifts the heat transfer mechanism to forced convection, where fluid flow is driven by an external factor (the vehicle's motion). The convective heat transfer coefficient (\(h\)) scales strongly with fluid velocity (\(v\)). For flow over a cylinder or body profile, this relationship is typically expressed using non-dimensional correlations such as: \[ Nu = \frac{h L}{k} = C \cdot Re^m \cdot Pr^n \quad \Rightarrow \quad h \propto v^m \quad (\text{where } m \approx 0.5 \text{ to } 0.8) \] As velocity increases, the boundary layer thins, which increases both the convective heat transfer coefficient (\(h_{\text{forced}} \gg h_{\text{natural}}\)) and the rate of sweat evaporation.

Step 3: Correlating to human thermal comfort.
The significant increase in forced convective heat transfer and evaporative cooling allows the body to reject heat much more rapidly while in motion, making the rider feel cooler and more comfortable. This matches Option (1).