Wall heat transfer of unsteady near-wall flow in internal combustion engines

Abstract

Although the near-wall turbulence is not fully developed in the engine combustion chamber, wall heat transfer models based on flow characteristics in fully developed near-wall turbulence are typically employed in engine simulations to predict heat transfer. Only few studies reported the wall heat transfer mechanism in near-wall flow where the near-wall turbulence was not fully developed as expected in the engine combustion chamber. In this study, the velocity distribution and wall heat flux in such a near-wall flow were evaluated using a rapid compression and expansion machine. In addition to the experimental approach, a numerical simulation with highly resolved calculation mesh was applied in various flow fields expected in the engine combustion chamber. As a result, the turbulent Reynolds number that represents the relationship between turbulent production and dissipation varied in the wall boundary layer according to the near-wall flow condition. This behavior affects the wall heat transfer. Considering this finding, a new model was formulated by introducing a ratio of turbulent Reynolds number in an intended near-wall flow to that in fully developed near-wall turbulence. It was confirmed that the proposed model could improve the prediction accuracy of wall heat flux even in near-wall flow where the near-wall turbulence was not fully developed. By applying the proposed model in engine computational fluid dynamics, it was found that the proposed model could predict the wall heat flux in a homogeneous charge compression ignition gasoline engine with acceptable accuracy.

A new concept for high efficiency and clean diesel combustion by controlling mixture distribution with dual zone combustion chamber

“…We applied 0.6 as c qw . For the friction velocity u τ , equation (2) proposed by Harada et al 22 was applied, where R e t and R e t _ dev denote the turbulent Reynolds number and that in the fully developed near-wall flow, respectively. C μ denotes a constant usually assumed as 0.09, and k means the turbulent kinetic energy in the wall boundary layer.…”

Special Issue on COMODIA 2017

“…The articles in this category include experimental and theoretical analysis of wall heat transfer, and a new approach to reduce heat loss in a compression ignition engine. In the first paper, Harada et al 8 propose a model based on Han and Reitz 9 to predict the wall heat flux even when the turbulent near-wall flow is not fully developed. The accuracy of the proposed model is evaluated in an HCCI (homogeneous-charge compressionignition) engine.…”

Infrared high-speed thermography of combustion chamber wall impinged by diesel spray flame

“…As a potential and practical remedies for further improvement of diesel engine thermal efficiency, reduction of cooling loss is attracting growing attention. [1][2][3][4][5] In a recent Japanese national project, Strategic Innovation Program (SIP) for Innovative Combustion Technology 6 completed in March 2019, notable thermal efficiency improvement of passenger vehicle diesel engine by cooling loss reduction has been demonstrated with highpressure split main injection, 7 inversed-delta injection rate shaping, 8 and early-evaporating compact spray using a two-component fuel. 9 However, the heat transfer from the diesel flame to the chamber wall is an extremely unsteady and spatially varying process.…”

“…The details of the process and its mechanism remain unclear and relevant engine heat transfer models are under development. 2,[10][11][12][13][14][15] In many of previous experimental investigations targeting elucidation of engine heat transfer, varieties of point-measurement heat flux sensors have been utilized. 9,[16][17][18][19][20] These sensors feature fast response, high accuracy, compactness and ruggedness.…”

“…The details of the process and its mechanism remain unclear and relevant engine heat transfer models are under development. 2,10–15…”