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Abstract
Optimization of a parallel flow gas-to-gas tubular micro heat exchanger with hot core and cold annulus fluid is numerically analyzed, considering the beneficial role of surface radiation. Operating and geometric parameters are varied for fixed overall mass flow rate and temperature of cold core fluid, to study the effects on the following performance parameters: heat transferred to annulus fluid, logarithmic mean temperature difference, effectiveness, and volumetric heat transfer coefficient. The micro heat exchanger is optimized for high heat transfer to annulus fluid and volumetric heat transfer coefficient, for different operating and geometric conditions. Optimization for high volumetric heat transfer coefficient maximizes the micro heat exchanger effectiveness, heat transferred and improves logarithmic mean temperature difference.
ACKNOWLEDGMENT
The corresponding author thanks IIT Bombay for granting deputation status at IIT Mandi, Himachal Province, India.
NOMENCLATURE
| A | = | surface area (m2) |
| B | = | radiance from surface (W/m2) |
| Bi | = | Biot number |
| C | = | heat capacity rate (W/K) |
| cp | = | specific heat at constant pressure (J/kg-K) |
| D | = | diameter (m) |
| Dh | = | hydraulic diameter (m) |
| dann | = | annular spacing between two coaxial cylinders in heat exchanger (m) |
| Fi-j | = | view factor of surface i as seen by surface j |
| H | = | irradiance incident on surface (W/m2) |
| hc | = | convective heat transfer coefficient (W/m2-K) |
| hV | = | volumetric heat transfer coefficient (W/m3-K) |
| k | = | thermal conductivity (W/m-K) |
| Kn | = | Knudsen number |
| L | = | length of heat exchanger (m) |
| LMTD | = | logarithmic mean temperature difference (°C, K) |
| M | = | Mach number |
| Nu | = | Nusselt number |
| = | mass flow rate (kg/s) | |
| MHEx | = | micro heat exchangers |
| N | = | number of axial elements in which heat exchanger is discretized |
| opt | = | optimum |
| p | = | pressure (Pa) |
| q | = | heat flow rate (W) |
| Q | = | total heat flow rate (W) |
| q″ | = | heat flux (W/cm2) |
| R | = | radius of pipe in heat exchanger (m) |
| ReD | = | Reynolds number based on diameter |
| Rg | = | specific gas constant (= 287.05 J/kg-K, for dry air) |
| t | = | thickness of inner pipe (1) (m) |
| T | = | temperature (°C, K) |
| Tfm | = | bulk mean fluid temperature (°C, K) |
| u | = | axial flow velocity (m/s) |
| V | = | volume of heat exchanger (m3) |
| VHTC | = | volumetric heat transfer coefficient (W/m3-K) |
| z | = | flow direction (m) |
Greek Symbols
| ΔTm | = | logarithmic mean temperature difference (LMTD) (°C, K) |
| Δz | = | width of discretized element (m) |
| ϵ | = | surface emissivity |
| κ | = | Radius ratio |
| η | = | Effectiveness |
| μ | = | dynamic viscosity (Pa-s) |
| ρ | = | air density (kg/m3) |
| σ | = | Stefan–Boltzmann constant (= 5.6704×10−8 W/m2-K4) |
Subscripts
| cv | = | convective |
| ed | = | exit disk |
| ex | = | exit |
| f or fl | = | fluid |
| i | = | inner surface |
| id | = | circular disk of radius Ri at inlet of inner pipe (1) |
| in | = | inlet |
| k | = | kth element of discretized heat exchanger layout |
| m | = | mean value over cross section |
| max | = | maximum value |
| min | = | minimum value |
| o | = | outer surface |
| rad | = | radiative |
| T | = | net value for inner/outer surface of cylinder |
| w | = | wall |
| 0 | = | stagnation/total value |
| 1 | = | inner pipe/airflow through inner pipe |
| 2 | = | outer pipe/airflow through annulus |
| ∞ | = | ambient value |
Additional information
Notes on contributors
K. N. Yogish
K. N. Yogish is a master's student in the Department of Aerospace Engineering, Indian Institute of Technology Bombay (IIT-B), India. He is a graduate in mechanical engineering. His areas of interest include micro convective flow, micro heat exchangers, and infrared signature studies of aerospace vehicles.
Shripad P. Mahulikar
Shripad P. Mahulikar is a professor (on deputation) in the School of Engineering, Indian Institute of Technology Mandi, Himachal Province, India. He is a permanent professor in the Department of Aerospace Engineering, Indian Institute of Technology Bombay (IIT-B). He obtained his B.Tech. and M.Tech. (by research) in aerospace engineering from IIT-B in 1990 and 1992, respectively, and earned his Ph.D. from Nanyang Technological University, Singapore, in 1999. He received the A. von Humboldt Fellowship in the Federal Republic of Germany (in 2003, 2007, 2009), Outstanding Reviewer Award from the ASME Journal of Heat Transfer in 2007, and DFG-Mercator Professorship in Hamburg University of Technology, Federal Republic of Germany (December 2011 onward).