Question

Determine the hydrodynamic and thermal entry lengths for water, engine oil, and liquid mercury flowing through a $2.5$-cm-diameter smooth tube with mass flow rate of $0.01\mathrm{kg}/\mathrm{s}$ and temperature of ${100}^{\circ }\mathrm{C}$.

Short answer

Question: Calculate the hydrodynamic and thermal entry lengths for water, engine oil, and liquid mercury flowing through a smooth tube, given their diameter (D), mass flow rate (ṁ), and temperature (T). Answer: To find the entry lengths, first calculate the Reynold's number (Re) and Prandtl number (Pr) for each fluid using their properties (density, viscosity, specific heat capacity, and thermal conductivity) at the given temperature. Then, based on the flow regime (laminar or turbulent), apply the appropriate formulae to calculate the hydrodynamic and thermal entry lengths. Finally, present the results for each fluid.

Step-by-step solution

Calculate the Reynold's number for each fluid

To calculate the Reynold's number, we first need to find the fluid velocity ($v$), using the mass flow rate and the fluid properties like density ($\rho$) and cross-sectional area ($A$) of the pipe. The formula for Reynold's number is: $Re=\frac{\rho vD}{\mu }$, where $\mu$ is the dynamic viscosity. For water, engine oil, and mercury, we will use the following density and viscosity values at ${100}^{\circ }C$: Water: ${\rho }_w=958\mathrm{kg}/{\mathrm{m}}^3$ and ${\mu }_w=2.82\times {10}^{-4}\mathrm{Pa}\cdot \mathrm{s}$ Engine oil: ${\rho }_e=872\mathrm{kg}/{\mathrm{m}}^3$ and ${\mu }_e=1.29\times {10}^{-3}\mathrm{Pa}\cdot \mathrm{s}$ Mercury: ${\rho }_m=13650\mathrm{kg}/{\mathrm{m}}^3$ and ${\mu }_m=2.25\times {10}^{-4}\mathrm{Pa}\cdot \mathrm{s}$ Compute fluid velocity and Reynold's number for each fluid.

Determine Prandtl number for each fluid

Prandtl number is calculated using the formula: $Pr=\frac{\mu c_p}{k}$, where $c_p$ is specific heat capacity and $k$ is the thermal conductivity. For water, engine oil, and mercury, we will use the following specific heat capacity and thermal conductivity values at ${100}^{\circ }C$: Water: $c_{p_w}=4.22\mathrm{kJ}/(\mathrm{kg}\cdot \mathrm{K})$ and $k_w=0.68\mathrm{W}/(\mathrm{m}\cdot \mathrm{K})$ Engine oil: $c_{p_e}=2.1\mathrm{kJ}/(\mathrm{kg}\cdot \mathrm{K})$ and $k_e=0.15\mathrm{W}/(\mathrm{m}\cdot \mathrm{K})$ Mercury: $c_{p_m}=0.139\mathrm{kJ}/(\mathrm{kg}\cdot \mathrm{K})$ and $k_m=8.55\mathrm{W}/(\mathrm{m}\cdot \mathrm{K})$ Compute Prandtl number for each fluid.

Calculate the entry lengths

Based on the Reynold's number calculated in Step 1, determine if the flow regime is laminar or turbulent. If $Re<2000$, it's laminar, and if $Re>4000$, it's turbulent. For each fluid, apply the appropriate formulae for hydrodynamic and thermal entry lengths mentioned in the analysis using $Re$, $Pr$, and $D$. Once all the entry lengths are calculated, present the results for each fluid.

Key concepts

Reynolds Number Calculation

The Reynolds number (Re) is a dimensionless quantity in fluid mechanics that helps predict flow patterns in different fluid flow situations. It plays a critical role in determining whether the flow will be laminar or turbulent.

To calculate the Reynolds number, you use the formula: $Re=\frac{\rho vD}{\mu }$, where $\rho$ is the density of the fluid, $v$ is the mean velocity of the fluid, $D$ is the characteristic length (diameter of the pipe in this case), and $\mu$ is the fluid's dynamic viscosity. In the exercise, the diameters of the pipes for all fluids are the same, hence the characteristic length doesn't vary.

For an accurate Reynolds number calculation, it's imperative to consider the fluid properties at the given temperature, in this case, ${100}^{\circ }C$.Determining the correct phase of water, engine oil, or mercury and their respective properties at this temperature ensures an accurate Reynolds number, which dictates the flow regime.

Prandtl Number

The Prandtl number (Pr) is a dimensionless number, significant in heat transfer calculations. It represents a ratio of momentum diffusivity (viscosity) to thermal diffusivity and is given by the formula: $Pr=\frac{\mu c_p}{k}$ where $\mu$ is the dynamic viscosity, $c_p$ is the specific heat capacity, and $k$ is the thermal conductivity.

The specific properties needed for calculating the Prandtl number also depend on the fluid's temperature. This number gives insight into the relative thickness of the velocity boundary layer to the thermal boundary layer. For fluids with higher Prandtl numbers, the thermal diffusivity is lower, indicating that the thermal boundary layer is thinner compared to the velocity boundary layer. This is crucial when assessing the influence of fluid properties on heat transfer, especially when looking at the thermal entry length in heat exchange scenarios.

Fluid Mechanics for Heat Transfer

Understanding the relationship between fluid mechanics and heat transfer is essential for engineering applications, such as designing efficient heat exchangers. Two primary considerations in this domaine are hydrodynamic and thermal entry lengths.

Hydrodynamic entry length is the distance over which the flow develops from the pipe inlet into a fully developed, predictable pattern, while thermal entry length is the distance required for the thermal profile to become fully developed. If we visualize these concepts in a pipe, the farther down the flow travels, the more it becomes thermally uniform across the section.

Calculating these entry lengths involves understanding flow regimes, which depend on the Reynolds number, and heat transfer characteristics, which are influenced by the Prandtl number. The significance of these lengths lies in their role in determining where the heat transfer between the fluid and the pipe will be most effective—a critical element in thermal system design.

Laminar and Turbulent Flows

Fluid flow can be classified into two main types: laminar and turbulent. Laminar flow is characterized by smooth, orderly fluid motion, where all particles move in parallel paths. Turbulent flow, on the other hand, is chaotic, with particles moving in random directions and speeds.

The flow type is determined by the Reynolds number. A low Reynolds number, typically less than 2000, indicates a laminar flow, whereas a high Reynolds number, generally above 4000, signifies turbulent flow. Transitional flow occurs in the range between these two extremes.

When considering hydrodynamic and thermal entry lengths, laminar flows tend to have longer hydrodynamic entry lengths compared to turbulent flows, due to the more orderly energy distribution. The thermal entry lengths are also significantly affected by whether the flow is laminar or turbulent. In turbulent flows, the mixing effect leads to a rapid development of the thermal profile, resulting in shorter thermal entry lengths. These flow characteristics heavily influence the performance and design of systems involving fluid motion and heat transfer, such as cooling systems or chemical reactors.