RE: [EnergyPlus_Support] Problems with Zone:VentilationLWindandStackOpenArea

This is basically correct. The convection models that depend on ACH get the flow rate from the mass flow at the zone’s air system node which is driven by *mechanical* air systems. The *simple* air flow models for ventilation and infiltration are separate from mechanical ventilation and are not captured in that ACH for convection models. Those surface convection models that depend on system air flow rate are also expecting a room air flow situation that is typical for room air distribution designed with directional air diffusers and the like. But non-mechanical ventilation and infiltration air movement is not going to have the same movement pattern, or even any predictable pattern, and there is a lack of simple ACH-dependent correlations to use for that situation. It isn’t clear that one could just add the non-mechanical ACH to the mechanical ACH without abusing the intent of convection models developed for another purpose. The “fix” would involve research and development of new correlations.

You can input your own custom models for surface convection that apply to your situation (SurfaceConvectionAlgorithm:Inside:UserCurve).

There is a minimum limit on convection coefficient values that can be controlled on input (in HeatBalanceAlgorithm object). Low temperature differences often lead quite small values for the coefficient, in my opinion it is useful to set the minimum limit fairly high, say 1.0 W/m2-K or even higher.

From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx] On Behalf Of HJ
Sent: Friday, January 04, 2013 9:41 AM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: Re: [EnergyPlus_Support] Problems with Zone:VentilationLWindandStackOpenArea

I have checked the engineering manual and it says adaptive model should incorporate ACH when mechanical ventilation is used and I checked the results and it does. However, when I use Zone:VentilationLWindandStackOpenArea to study natural ventilation, the ACH is not used. The convective heat transfer is important for calculating the heat transfer between wall and air, thus will dramatically influence the energy use when hybrid ventilation is used. I often find the E+ value of convective heat transfer coefficient is almost half or even smaller than the common value. I wonder if there is a way to fix this.

On 12/19/2012 1:30 AM, jeannieboef wrote:

No. I don't think the convection heattransfer algorithims incorporate ACH (I've not checked the engineering reference manual). the internal air flow speed in an e+ zone is set once in a People object for the zone and only used for comfort calculations.

My impression is that convection algorithms will use free flow (not driven) convection corrillations. These are well corrolated for typical flat surface geometries.

You make a valid point. This is the realm of CFD.

Sent from my iPhone

On 18.12.2012, at 20:40, HJ <wanghaojie630@xxxxxxxxx> wrote:

Hi all,

I try to simulate nightcooling by using
Zone:VentilationLWindandStackOpenArea and by scheduling a window
operation based on outdoor temperature and other parameters. For the
SurfaceConvectionAlgorithmInside, I use adaptive model. To my
understanding, adaptive model will include the impact of ACH on
convective heat transfer, thus I expect when the window is opened, the
convective heat transfer should be larger compared to when the window is
closed (No other mechanical system is on when the window is closed so
the zone ACH should be zero) However, as I read the data, the convective
heat transfer does not change much at the time when the window start to
be opened and I can see that the Zone ACH is not zero. So can anybody
tell me is there anything that I did wrong or this zone ventilation
model is separated from the SurfaceConvection model?
Also, there is an input of opening area for this
Zone:VentilationLWindandStackOpenArea, if this value is not zero, will
this infect the wall area of the building for such zone? (Real wall
area= wall area- opening area) Since I already draw the windows in
Sketchup, and still need to define the opening area here within this model.

Many thanks,

HJ

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