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ABSTRACT
Enhanced heat transfer tubes (EHTT) with segmented mesh-conical frustums are considered. Tube diameter and frustum apex angle are fixed as 20 mm and 60o, respectively. The height ratio of frustum and sliced part are set as a golden ratio (1.618). Laminar thermal-hydraulic performance and effects of some parameters, e.g., bottom frustum diameter and pitch, are numerically simulated. The equal equivalent diameter and total flow area criteria are adopted to simplify 3D mesh pores to 2D ones. Flow and temperature fields show large velocities and gradients close to the wall and smaller velocities in the bulk region. This enhances heat transfer with a limited pressure drop. EHTTs obtain 1.4 - 3.3 times higher heat transfer than bare tubes and the performance evaluation criterion (PEC) varies from 1.3 to 1.8. Nusselt number (Nu) and friction factor (f) correlations are proposed. New insights into heat transfer enhancement and tube configuration are provided.
Nomenclature
| A | = | flow area of total 3D mesh pore, m2 |
| A′ | = | flow area of total 2D mesh pore, m2 |
| BT | = | bare tube without insert |
| Cp | = | specific heat, J/(kg · K) |
| D | = | tube diameter, m |
| De | = | equivalent diameter of 3D mesh pores |
| = | equivalent diameter of 2D mesh pores | |
| EF | = | heat transfer enhancement factor |
| EHTT | = | enhanced heat transfer tube |
| f | = | friction factor |
| L1 | = | the distance from the tube entrance to the first mesh conical frustum insert, m |
| L2 | = | heat transfer tube length with consecutive mesh inserts, m |
| m | = | mass flow rate, kg/s |
| Nu | = | average Nusselt number (or called bulk Nusselt number) |
| Nux | = | local Nusselt number |
| p | = | pressure, Pa |
| Δp | = | pressure drop in a periodic unit, Pa |
| PEC | = | performance evaluation criterion |
| PPI | = | the specification of mesh screen (pores per inch) |
| q | = | heat flux, W/m2 |
| qw | = | the heat flux added to the wall, W/m2 |
| r | = | radial coordinate, m |
| Re | = | Reynolds number |
| S | = | periodic unit length, m |
| Tf | = | fluid temperature, K |
| Tf,ave | = | average fluid temperature, K |
| Tgrid | = | temperature within the first fluid layer, K |
| Tin | = | inlet fluid temperature, K |
| Tout | = | outlet fluid temperature, K |
| Tup.f | = | bulk fluid temperature at inlet, K |
| Tw | = | wall temperature, K |
| Tw,ave | = | average wall temperature, K |
| u | = | axial velocity, m/s |
| uin | = | inlet velocity, m/s |
| uave | = | average velocity, m/s |
| w | = | square mesh pore width, m |
| w′ | = | stripe-type mesh pore width, m |
| x | = | axial coordinate, m |
| y | = | first fluid layer thickness, m |
| α | = | the apex angle of the mesh conical frustum |
| β | = | pressure gradient within a periodic unit length, Pa/m |
| δ | = | the bottom diameter of the frustum, m |
| η | = | 3D mesh wire thickness, m |
| η′ | = | 2D mesh wire thickness, m |
| θ | = | temperature gradient, K/m |
| λ | = | thermal conductivity, W/(m · K) |
| µ | = | dynamic viscosity, Pa · s |
| ρ | = | density, kg/m3 |
| Subscript | = | |
| ave | = | average |
| b | = | bare tube |
| f | = | fluid |
| in | = | inlet |
| out | = | outlet |
| w | = | wall |
| x | = | axial coordinate |
Nomenclature
| A | = | flow area of total 3D mesh pore, m2 |
| A′ | = | flow area of total 2D mesh pore, m2 |
| BT | = | bare tube without insert |
| Cp | = | specific heat, J/(kg · K) |
| D | = | tube diameter, m |
| De | = | equivalent diameter of 3D mesh pores |
| = | equivalent diameter of 2D mesh pores | |
| EF | = | heat transfer enhancement factor |
| EHTT | = | enhanced heat transfer tube |
| f | = | friction factor |
| L1 | = | the distance from the tube entrance to the first mesh conical frustum insert, m |
| L2 | = | heat transfer tube length with consecutive mesh inserts, m |
| m | = | mass flow rate, kg/s |
| Nu | = | average Nusselt number (or called bulk Nusselt number) |
| Nux | = | local Nusselt number |
| p | = | pressure, Pa |
| Δp | = | pressure drop in a periodic unit, Pa |
| PEC | = | performance evaluation criterion |
| PPI | = | the specification of mesh screen (pores per inch) |
| q | = | heat flux, W/m2 |
| qw | = | the heat flux added to the wall, W/m2 |
| r | = | radial coordinate, m |
| Re | = | Reynolds number |
| S | = | periodic unit length, m |
| Tf | = | fluid temperature, K |
| Tf,ave | = | average fluid temperature, K |
| Tgrid | = | temperature within the first fluid layer, K |
| Tin | = | inlet fluid temperature, K |
| Tout | = | outlet fluid temperature, K |
| Tup.f | = | bulk fluid temperature at inlet, K |
| Tw | = | wall temperature, K |
| Tw,ave | = | average wall temperature, K |
| u | = | axial velocity, m/s |
| uin | = | inlet velocity, m/s |
| uave | = | average velocity, m/s |
| w | = | square mesh pore width, m |
| w′ | = | stripe-type mesh pore width, m |
| x | = | axial coordinate, m |
| y | = | first fluid layer thickness, m |
| α | = | the apex angle of the mesh conical frustum |
| β | = | pressure gradient within a periodic unit length, Pa/m |
| δ | = | the bottom diameter of the frustum, m |
| η | = | 3D mesh wire thickness, m |
| η′ | = | 2D mesh wire thickness, m |
| θ | = | temperature gradient, K/m |
| λ | = | thermal conductivity, W/(m · K) |
| µ | = | dynamic viscosity, Pa · s |
| ρ | = | density, kg/m3 |
| Subscript | = | |
| ave | = | average |
| b | = | bare tube |
| f | = | fluid |
| in | = | inlet |
| out | = | outlet |
| w | = | wall |
| x | = | axial coordinate |
Acknowledgment
Financial supports from the Swedish Research Council (VR) and China Scholarship Council (CSC) are kindly acknowledged.