Using KeywordQuery In sharepoint

If you want to limit the KeywordQuery to current site only, use the HiddenConstraints property.
Following is an example:

string siteName = SPContext.Current.Site.RootWeb.Title;

SearchServiceApplicationProxy proxy = (SearchServiceApplicationProxy)SearchServiceApplicationProxy.GetProxy

       (SPServiceContext.GetContext(SPContext.Current.Site));

            KeywordQuery query = new KeywordQuery(proxy);

            query.ResultsProvider = Microsoft.Office.Server.Search.Query.SearchProvider.Default;

            query.QueryText = queryText;

            query.HiddenConstraints = "This Site: " + siteName + "";

            query.ResultTypes |= ResultType.RelevantResults;

References:

http://sharepointnadeem.blogspot.sg/2012/09/limit-keywordquery-to-current-site-in.html

http://social.technet.microsoft.com/wiki/contents/articles/18202.sharepoint-2010-search-using-the-keywordquery-class.aspx

http://technologybooth.blogspot.sg/2013/08/keyword-query-in-sharepoint-2013.html