Question 1

Radiative properties of media: which of the following are generally considered opaque to thermal radiation over typical engineering thicknesses?

Accepted Answer

Introduction: Opacity describes whether a medium allows radiation to pass through. In heat transfer practice, most common solids and liquids are treated as opaque to thermal radiation, whereas many gases are partially transparent over engineering path lengths, with absorption in specific spectral bands. Given Data / Assumptions: Thermal radiation (infrared) wavelengths of interest. Typical engineering thicknesses (millimeters to centimeters). Homogeneous, continuum media behavior. Concept / Approach: For radiation analysis, surfaces of solids and free surfaces of liquids generally act as opaque boundaries with emissivity/absorptivity defining surface exchange. Gases like CO2 and H2O vapor absorb selectively but are often semi-transparent; path length and concentration determine their effective opacity. Hence, as a broad classification, solids and liquids are “opaque,” gases are not necessarily so. Step-by-Step Solution: Classify phases by typical radiative behavior.Recognize solids and liquids are treated as opaque in most designs.Select “Both (b) and (c)” accordingly. Verification / Alternative check: Standard radiation texts model solids/liquids as gray, opaque surfaces, while participating media treatment is reserved for gases/flames and certain semitransparent solids (e.g., glass at specific wavelengths). Why Other Options Are Wrong: Gases: Often semi-transparent; not broadly opaque. Individual (b) or (c): Each true, but the combined answer reflects both phases. Common Pitfalls: Overgeneralizing: some solids (e.g., thin glass) can be semitransparent in parts of the spectrum, but the engineering default for thermal analysis is opacity. Final Answer: Both (b) and (c)

Question 2

Kirchhoff’s law of thermal radiation relates emissivity and absorptivity at thermal equilibrium.\r To which type(s) of radiation does this law apply?

Accepted Answer

Introduction: Kirchhoff’s law links how a surface emits and absorbs radiation. At a given temperature and wavelength, a body’s emissivity equals its absorptivity provided the body is in thermodynamic equilibrium with its surroundings. This principle underpins the use of gray and selective surface models in engineering calculations. Given Data / Assumptions: Thermal equilibrium between surface and radiation field. Definitions of emissivity and absorptivity for both monochromatic (spectral) and total quantities. Diffuse behavior assumed unless directionality is specified. Concept / Approach: Kirchhoff’s law is most precisely stated spectrally: at each wavelength and direction, emissivity equals absorptivity. Integrating over all wavelengths yields the total (hemispherical) version. Therefore, it holds for monochromatic and for total radiation, provided the equilibrium condition is met and the same temperature is considered. Step-by-Step Solution: State spectral form: ε_λ = α_λ at the same λ and T.Integrate over λ to obtain total form: ε = α under equilibrium.Hence, the correct choice encompasses both monochromatic and total radiation. Verification / Alternative check: Blackbody reference cavities demonstrate the equality, while selective coatings show wavelength-dependent behavior consistent with the spectral statement. Why Other Options Are Wrong: Only total or only monochromatic: Each is incomplete; the law is valid in both domains when stated appropriately. Neither: Contradicts fundamental radiative transfer theory. Common Pitfalls: Forgetting the equilibrium requirement; surfaces exchanging with non-equilibrium fields may not satisfy the equality without correction. Final Answer: both (a) & (b)

Question 3

Colburn j-factor for heat transfer (j_H): as temperature increases for a typical fluid in turbulent flow, how does the j_H factor usually vary, considering the concurrent change in fluid properties (notably Prandtl number)?

Accepted Answer

Introduction: The Colburn j-factor provides a convenient way to correlate convective heat transfer data: j_H = St * Pr^(2/3). It ties heat transfer to momentum transfer and is widely used in exchanger rating and air-side correlations. Because j_H contains Pr^(2/3), variations in temperature that alter viscosity and thermal diffusivity will influence its value. Given Data / Assumptions: Turbulent forced convection with property variations evaluated at film temperature. Increasing temperature usually reduces viscosity and Prandtl number for many liquids and gases. Flow rate (velocity) held fixed for comparison; Re may rise if μ drops, but property corrections are applied consistently. Concept / Approach: Since j_H = St * Pr^(2/3) and, for a given geometry, St correlates primarily with Re, a drop in Pr with temperature tends to reduce j_H. Even though Re may increase as μ decreases, the explicit Pr^(2/3) dependence usually dominates the j_H trend in standard charts, leading to a net decrease with temperature for common fluids over practical ranges. Step-by-Step Solution: Note typical property trend: as T rises, μ ↓ and Pr = ν/α often ↓.Recognize j_H’s explicit Pr^(2/3) multiplier.Infer the overall tendency: j_H decreases with rising temperature. Verification / Alternative check: Air- and water-side j-factor charts display lower j-values at higher film temperatures for otherwise similar conditions, consistent with Prandtl decreases. Why Other Options Are Wrong: Increases/remains unchanged: Contradict typical property-driven behavior. May increase or decrease: While special cases exist, the question asks the usual trend; decrease is the standard expectation. Oscillation with Re: j_H varies smoothly with Re; no periodic oscillation. Common Pitfalls: Confusing j_H with Nusselt number alone; property grouping in j_H makes temperature trends more apparent. Final Answer: decreases.

Question 4

Continuous crystallizer design: crystal size distribution in a well-mixed crystallizer is governed by multiple interacting factors.\r Which set of variables collectively influences the resulting crystal size?

Accepted Answer

Introduction: Crystal size distribution (CSD) is a key quality attribute in pharmaceutical, food, and inorganic chemical processes. It affects downstream filtration, drying, and product performance. In continuous crystallizers, both nucleation and growth kinetics are shaped by operating conditions and hydrodynamics. Given Data / Assumptions: Nucleation depends on supersaturation (driving force). Growth and breakage are affected by mixing and shear. Heat removal controls how quickly supersaturation is created or relieved. Concept / Approach: Higher supersaturation promotes nucleation (more small crystals), while controlled, lower supersaturation favors growth (larger crystals). Turbulence influences micromixing, secondary nucleation, and breakage/agglomeration. Heat transfer regulates the creation of supersaturation (e.g., cooling or evaporative duty). Hence, crystal size is a function of all three: heat transfer rate, turbulence level, and supersaturation magnitude. Step-by-Step Solution: Link heat duty to supersaturation generation.Relate turbulence to secondary nucleation and attrition.Recognize supersaturation as the primary thermodynamic driving force.Conclude that all listed factors jointly determine size. Verification / Alternative check: Population balance models explicitly include nucleation, growth, aggregation, and breakage terms, each sensitive to these variables. Why Other Options Are Wrong: Any single factor alone is incomplete; CSD emerges from multiple coupled effects. Feed elevation is hydraulics, not a direct determinant of CSD. Common Pitfalls: Attempting to tune only temperature without considering mixing; this leads to unstable CSD and operational fouling. Final Answer: all (a), (b) and (c).

Question 5

Shell-and-tube exchanger hydraulics: the pressure drop on the shell side depends on geometry and fluid properties.\r Which option correctly reflects this dependency?

Accepted Answer

Introduction: Predicting shell-side pressure drop is essential to ensure adequate pumping power and to avoid tube vibration or bypassing. The drop arises from crossflow over tube banks, window flow through baffles, leakage streams, and bypass channels—all influenced by hardware dimensions and fluid properties. Given Data / Assumptions: Typical 1–2 or 2–4 shell-and-tube configurations with segmental baffles. Crossflow patterns governed by baffle spacing, cut, and sealing. Fluid properties evaluated at bulk or film temperatures. Concept / Approach: Empirical methods (e.g., Bell–Delaware) show shell-side pressure drop depends on baffle spacing and shell diameter (flow area and path), tube layout (diameter, pitch, pattern), and fluid properties (viscosity, density) together with mass velocity. Therefore, every factor listed in (a), (b), and (c) affects the pressure drop; it is not independent of them. Hence, “none of these” is the correct selection to the phrased question “does not depend upon.” Step-by-Step Solution: List geometric factors: shell size, baffle spacing, tube pitch/diameter.List property/operating factors: ρ, μ, mass velocity G.Conclude dependence on all; therefore none can be excluded. Verification / Alternative check: Correlations compute pressure drop using correction factors for leakage, bundle bypassing, and unequal baffle spacing—all tied to geometry and properties. Why Other Options Are Wrong: (a), (b), (c): Each undeniably influences shell-side pressure drop. (e): Baffle cut matters, but it is not the only parameter. Common Pitfalls: Overlooking sealing strips or excessive baffle spacing—both can reduce heat transfer and alter pressure drop undesirably. Final Answer: none of these.

Question 6

Foundations of heat transfer in engineering: Fourier's law applies specifically to which mode of heat transfer in solids and stationary fluids?

Accepted Answer

Introduction / Context: Heat can move by three fundamental modes: conduction, convection, and radiation. Each mode has its own governing law and constitutive relation. This question tests recognition of which mode is described by Fourier's law, a cornerstone in thermal analysis of solids and quiescent media. Given Data / Assumptions: Steady or unsteady heat flow in a continuum medium. Local thermodynamic equilibrium so that temperature is well-defined. No bulk fluid motion for the constitutive relation itself (i.e., not a convective transport law). Concept / Approach: Fourier's law states that the heat flux vector q is proportional to the negative temperature gradient: q = −k * grad(T). The thermal conductivity k is a material property (possibly temperature-dependent). This law describes conduction only. Convection heat transfer is described by Newton's cooling law (q' = h * ΔT), while radiation follows the Stefan–Boltzmann law and view-factor relations (q' = εσ(T_s^4 − T_sur^4)). Step-by-Step Solution: Identify the law: Fourier's law connects heat flux to temperature gradient via conductivity.Recognize that it does not include a convective heat-transfer coefficient h or radiative terms like σ, ε.Therefore, it characterizes conduction in solids and stationary fluids.Select 'Conduction' as the only correct option. Verification / Alternative check: In a plane wall, one-dimensional steady conduction reduces Fourier's law to q' = k * (T_hot − T_cold) / L, matching textbook derivations. Neither Newton's law of cooling nor Stefan–Boltzmann relations can produce this form without additional assumptions. Why Other Options Are Wrong: Convection depends on fluid motion and boundary layers; it is summarized by Newton's law, not Fourier's. Radiation depends on T^4 behavior and surface exchange; Fourier's law contains no emissivity or radiative terms. 'All of these' is incorrect because Fourier's law is not the governing relation for convection or radiation. Common Pitfalls: Confusing conduction within a fluid with convective transport across a boundary layer. Assuming Fourier's law applies to composite mechanisms without superposition rules. Final Answer: Conduction

Question 7

Electrical heating (Joule effect): When a steady current I passes through a conductor of resistance R, the rate of heat generation (power) in the conductor equals:

Accepted Answer

Introduction / Context: Electrical resistance converts electrical energy into thermal energy, a phenomenon exploited in heaters and observed as I^2R losses in power systems. This problem asks for the correct expression for heat generation rate in a resistive element under steady current. Given Data / Assumptions: Steady direct current I (no time-varying fields). Ohmic conductor with resistance R (temperature effects neglected). Power converted to heat equals electrical input power for a pure resistor. Concept / Approach: From circuit theory, power P dissipated in a resistor can be written as P = V * I. Using Ohm's law, V = I * R, so P = I * (I R) = I^2 R. Equivalently, P = V^2 / R if voltage is known. The dimensional consistency and widespread engineering use of I^2 R solidify this as the correct form. Step-by-Step Solution: Start with P = V * I (definition of electrical power).Use Ohm's law: V = I * R.Substitute: P = I * (I * R) = I^2 * R.Therefore, heat generation rate equals I^2 R. Verification / Alternative check: Energy balance over time: Q = ∫ P dt = I^2 R * t for constant I, matching calorimetric measurements in resistive heating. Why Other Options Are Wrong: I R and I R^2 have incorrect units for power and are inconsistent with Ohm's law. I^2 R^2 overstates dependence on R and is dimensionally incorrect for power. Common Pitfalls: Mixing up P = I^2 R and P = V^2 / R; both are correct, but the given data favor I^2 R. Ignoring temperature rise changing R; here R is treated as constant. Final Answer: I^2 R

Question 8

Shell-and-tube heat exchanger design statements: Pick the incorrect statement from the following commonly cited design facts.

Accepted Answer

Introduction / Context: Shell-and-tube heat exchangers employ various mechanical features such as baffles, head types, and tube layouts to control heat transfer and pressure drop. Some statements about these features are accurate while others are misleading or outright false. Given Data / Assumptions: Conventional shell-and-tube exchanger terminology is used. Focus is on typical industrial practice (segmental, orifice/rod, and no-tubes-in-window options). Evaporators may be configured horizontally or vertically with air venting provisions. Concept / Approach: Determine which statement contradicts accepted practice. Orifice (disc-and-doughnut) baffles are indeed used where low shell-side pressure drop and gentle crossflow are desired. Tube pitch affects shell-side flow area and thus pressure drop. Air blanketing in horizontal-tube evaporators is a known concern and is mitigated by proper venting, not inherently “avoided.” Floating head types such as split-ring (S-type) and pull-through (P-type) are standard. Step-by-Step Solution: Evaluate (a): Claiming orifice baffles are “never” used is false; they are used for specific duties.Evaluate (b): True—tube pitch influences crossflow area and pressure drop.Evaluate (c): Misleading as written; air blanketing is an issue, so it is not “avoided” by default.Evaluate (d): True—both listed floating head types are common.Therefore, the incorrect statement is (a). Verification / Alternative check: Design manuals include orifice baffles and rod-baffle systems as recognized variants for fouling-sensitive or low ΔP services. Why Other Options Are Wrong: (b) is correct: geometry (pitch/layout) alters shell-side hydraulics. (c) as a blanket claim is not correct; however, the question requests a single incorrect statement, and (a) is definitively false. (d) is correct, describing standard TEMA floating head choices. Common Pitfalls: Treating “never” or “always” statements as true in engineering contexts. Confusing mitigation (venting) with inherent avoidance of air blanketing. Final Answer: Orifice baffles are never used in a shell-and-tube heat exchanger.

Question 9

Heating low-heat-transfer-coefficient gases: Air is most effectively heated with steam in which heat-exchanger configuration, considering where fins provide the greatest benefit?

Accepted Answer

Introduction / Context: When heating a gas such as air with steam, the controlling resistance is usually on the gas side because gases have much lower film heat-transfer coefficients than condensing steam. Fins multiply surface area and are most beneficial on the side with the smaller h-value. Given Data / Assumptions: Steam condenses on one side, air is heated on the other. Overall heat-transfer coefficient is dominated by the gas-side film resistance. Fins improve performance primarily by increasing area on the limiting side. Concept / Approach: To maximize overall U, increase area where h is lowest. Steam-side condensation typically has high h (hundreds to thousands W/m^2·K), while air-side h is small (tens W/m^2·K). Therefore, placing fins on the air side yields the largest reduction in total thermal resistance. Step-by-Step Solution: Identify limiting side: air side has low h.Apply finned surface where it matters most: the air side.Among listed options, choose double-pipe with fins on air side.Note that plate or shell-and-tube can also heat air, but without air-side fins they are less effective per unit size. Verification / Alternative check: Thermal circuits show a significant drop in overall resistance when fin efficiency and area ratios are applied to the gas side; steam-side finning often adds little where condensation already provides high h. Why Other Options Are Wrong: Fins on the steam side yield minimal benefit compared with finning the air side. Plate and shell-and-tube designs can be used, but without targeted air-side finning they typically require larger surface areas. Common Pitfalls: Adding fins where h is already high (steam side) and expecting large gains. Ignoring fin efficiency and fouling factors on dusty air streams. Final Answer: Double-pipe exchanger with fins on the air side

Question 10

Hydraulic versus thermal equivalent diameter: In non-circular flow passages and tube bundles, the equivalent diameter used for pressure-drop calculations is typically __________ the equivalent diameter used for heat-transfer correlations.

Accepted Answer

Introduction / Context: Engineers often use an “equivalent” or “hydraulic” diameter to apply circular-tube correlations to non-circular passages (ducts, annuli, and shell-side crossflow through tube bundles). Depending on how flow area and wetted perimeter are defined, the effective diameters for heat transfer and for frictional pressure drop can differ. Given Data / Assumptions: Crossflow over tube bundles or non-circular ducts where minimum free-flow area matters. Heat-transfer De may be based on overall wetted perimeter; pressure-drop De frequently references minimum flow area between obstructions. Same physical geometry but different correlation requirements. Concept / Approach: For many exchanger geometries, especially on the shell side, the equivalent diameter selected for pressure-drop correlation accounts for the minimum cross-sectional flow area, which is smaller than the area basis often used for heat-transfer De. As a result, De(pd) < De(ht) in common design handbooks, reflecting more restrictive passages that govern friction losses. Step-by-Step Solution: Define De(ht) and De(pd) per geometry-specific standards.Recognize that friction is sensitive to constrictions; heat transfer tends to average over wetted surfaces.Therefore, De for pressure drop is typically smaller than that for heat transfer.Select 'smaller than'. Verification / Alternative check: Shell-side design methods (e.g., TEMA/standards) provide distinct De expressions; example tables often show the pressure-drop De computed with minimum flow area, yielding a smaller value than the De used in heat-transfer j-factor correlations. Why Other Options Are Wrong: 'Equal to' disregards the differing definitions. 'Greater than' contradicts typical minimum-area basis for friction. 'Not related to' is incorrect; both are related but not identical. Common Pitfalls: Using a single De for both phenomena, leading to mispredicted ΔP or U. Ignoring baffle cut, pitch, and pattern effects on free-flow area. Final Answer: smaller than

Heat Transfer Questions